She said it almost as a confession, the way people do when they suspect the fault is theirs. She had done everything the wellbeing emails told her to do. Off the caffeine by two. Phone out of the bedroom. Eight hours ringed on the tracker like a target hit. And still she surfaced each morning already behind, already braced, as though the night had happened to someone else and left her the bill.

She wanted to know what was wrong with her sleep.

Nothing was wrong with her sleep. She was asking the wrong organ.

Here is the thing we get backwards. We treat tiredness as one substance, a single tank that empties through the day and refills overnight. Work drains it, rest restores it, and if rest stops restoring it, the rest must be broken. Fix the sleep, fix the tiredness. It is a clean model. It is also, past a certain point, wrong.

There are two kinds of tired, and they run on different circuits.

The first kind is the honest one. You worked hard, your body spent itself, and sleep pays it straight back. A long shift, a bad night on call, a week that asked too much. This is recoverable fatigue, and a full night genuinely mends it. You wake up and the tank is full again. Most people, most of the time, are living inside this kind. It behaves exactly as the model predicts.

The second kind does not behave. It is not about how hard your body worked. It is about how long your mind has been running without margin. And it does not drain the way muscles drain. It accumulates the way a debt does, quietly, with interest, in a currency that sleep cannot pay.

I have come to call the thing being spent Cognitive Oxygen® (COx). It is the margin that good thinking runs on: the steadiness, the attention, the few seconds between a stimulus and your response, the bandwidth to hold more than one thing in mind without dropping the lot. When that margin stays below what the role demands for long enough, you cross into something with its own physiology. Cognitive hypoxia. The state where the supply has fallen under the threshold the job requires, and stayed there.

And here the two kinds separate cleanly. The first is a shortfall in the tank: your body spent itself, and sleep refills it. The second is a mismatch in the wiring. The deciding parts of the brain, the frontal networks that weigh and plan and hold a second opinion, are the most fuel-hungry tissue you own and the first to feel a shortage. Ask them to run at full demand, day after day, with no slack, and they fall behind their own supply. That is not a tank you can top up. It is a load you have to redesign.

And cognitive hypoxia does not clock off at night.

Think about what your registrar was actually carrying to bed. Not tired legs. A nervous system that never got the signal to stand down. The decision she was second-guessing from the ward round. The conversation she rehearsed and never had. The forty small judgements she made at speed, under pressure, without a moment to set any of them down and look at them properly. All of it still running, low and constant, like a fan that will not switch off. She slept. Her body rested. But the part of her that decides, that weighs, that stays open to being wrong, never left the building.

That is why the morning felt borrowed. Sleep restored the muscle. It could not restore the margin, because the margin was never the sort of thing a night could refill. You cannot rest your way out of a debt you keep taking on before breakfast.

Notice what this does to the usual advice. Sleep hygiene, the ten-minute wind-down, the meditation app with the whale noises: all of it is aimed at the first kind of tired. It works, for that kind. But offer it to someone in cognitive hypoxia and you have handed a splint to a person who is short of breath. You have treated the wrong system, confidently, and when it fails to work they draw the obvious conclusion. The problem must be them. They are not resilient enough. They cannot even sleep properly.

They can sleep perfectly well. They are drowning in a different pool.

There is a reason this second tiredness bites hardest in people who decide for a living. Every unexamined call you carry, every assumption you never got to test out loud, every plan you shipped at speed and quietly hope holds, stays open in the background. This is the tax that Red Team Thinking® was built to collect deliberately, in daylight, on purpose, rather than letting it collect itself at three in the morning. Thinking that never gets externalised does not disappear. It goes underground and keeps charging you rent. The most stretched leaders I meet are not tired because they thought too hard. They are tired because they were never given the room to finish a thought, so none of it ever closed.

So the question to ask, when good sleep stops working, is not what is wrong with the night. It is:

What did I bring to bed that was never mine to hold alone?

And here is the part I find genuinely hopeful. You do not have to guess which tired you are in, and you do not have to wait for the car park at the end of the day to tell you. Both kinds leave a reading, and a reading can be watched. Set how you wake after a real night’s rest beside how loaded the day before actually was, week on week, two needles on one dashboard. When a good night moves the first needle back to where it should sit, that was recoverable fatigue behaving exactly as it should. When the needle stays flat after eight honest hours, that is the second kind speaking, and it is not asking you to sleep more. It is telling you the design of your days has outrun the margin your mind was built with.

The gap between the two readings is the diagnosis.

And a bad design, unlike a bad night, is something you are allowed to change.

Look at the capable person near you who is doing everything right and getting nowhere. Off their phone. On their tracker. Still grey by Thursday. Before you send them another link about sleep, sit with the possibility that their sleep is fine. The thing they cannot refill is not rest. It is margin. And margin does not come from a better bedtime routine. It comes from a system that stops taking the oxygen before the person has to go looking for it at midnight.

The registrar did not need a wind-down ritual. She needed ten minutes in the day where the decisions could be set down and examined, out loud, with someone, before they followed her home.

You do not sleep your way back from cognitive hypoxia. You think your way back, in the light, where the thoughts can be seen.

This week, when you wake up already tired, ask which tank is empty before you blame the night.

She was not. She ran a busy unit. She hit her numbers. Her patients did well and her juniors trusted her. From the outside, nothing was wrong. But every evening she sat in her car in the hospital car park for ten minutes before she could face the drive home. She had started snapping at people she loved. She read the same paragraph four times and took none of it in. She told me she had lost her edge, her patience, the version of herself she used to like.

She had not lost any of those things. She had run out of something. And because she could not name it, she blamed the only thing in the room she could see clearly. Herself.

We are good, in medicine, at naming what we can see. A rhythm on a monitor. A shadow on a film. A number that has drifted out of range. We are far less good at naming the thing that has no waveform, no image, no reference interval. So when a capable leader starts to struggle, we reach for the labels we already own.

We call it a leadership problem. We send them on a course about influence and difficult conversations.

We call it a communication breakdown. We commission a workshop, write a new policy, add a step to the handover.

We call it burnout, or a resilience gap, and we offer mindfulness at lunchtime and a wellbeing webinar nobody has time to watch.

None of it holds. The course is forgotten by the following Tuesday. The policy becomes another thing to comply with. The webinar is one more demand on a person who already has too many. We keep prescribing, and the patient keeps not getting better, and slowly we conclude that the patient is the problem.

Here is the reframe I want to leave you with this week.

Healthcare does not have a leadership crisis. It has an oxygen crisis.

I mean that almost literally. In medicine, oxygen is not one factor among many. It is the enabling condition. Get the oxygen wrong and it does not matter how strong the heart is or how clever the drug regime is. Everything downstream degrades. And the brain, the most oxygen-hungry organ we have, is the first to feel it. Starve it for seconds and higher function starts to fail before the person even knows anything is happening.

Leadership under modern pressure follows the same physiology. There is a kind of margin that good thinking runs on. Call it attention, emotional steadiness, the few seconds between a stimulus and your response, the bandwidth to hold more than one thing in mind without dropping the lot. When that margin is adequate, people are curious, steady, willing to be challenged, able to see the whole board. When it falls below what the role demands, the higher functions go first. Strategic thinking narrows to firefighting. Empathy shrinks. Curiosity collapses into certainty. The person becomes sharper, colder, quicker to close a conversation down.

This is the thing my colleague had run out of. I have come to call it Cognitive Oxygen® (COx), and I have spent years watching it deplete in people far too able to be failing.

The cruel part is how well the depletion hides. A starving heart does not stop. It compensates. It works harder, recruits reserve, holds the line, and from the outside it looks like it is coping right up until the moment it is not. Depleted leaders do the same. They go into a kind of permanent triage, and triage produces visible activity. Inboxes get cleared. Meetings get attended. The motion looks like competence. So the very thing that should alarm us reassures us instead. Everyone is busy, therefore everyone must be fine.

And there is the matter of shame, which keeps the whole thing in the dark. Naming your own depletion feels like confessing weakness, so people do not say it. They sit in the car park instead. They assume they are the only one. They are almost never the only one.

I want to be careful here, because this is where the usual story goes wrong. The usual story says: you are depleted, so you must look after yourself better. Sleep more. Be more resilient. The implication is that the problem lives inside you, and so does the fix.

That is the part I do not accept. The leaders I have watched struggle most were not the least intelligent, the least trained, or the least committed. They were the most stretched. Their environments had quietly removed the conditions that good thinking requires, and then asked them to think well anyway, and then, when the thinking suffered, offered them a webinar about themselves. The depletion was real. The diagnosis was aimed at the wrong target.

So the question is not “how do I cope better.” Coping better is what she had been doing for two years in that car park, and it was eating her alive. The question is the one we ask about any patient who is not improving on the treatment we chose. Did we get the diagnosis right?

Look again at the next capable person you see starting to fray. The one who has gone quiet, or short-tempered, or strangely certain about everything. Before you reach for the word leadership, or communication, or resilience, try a different question.

What in this system keeps taking the oxygen out of the room?

Ask that, and a great deal changes. The problem stops being a person and starts being a set of conditions. Conditions can be redesigned. And an oxygen crisis, once you actually name it as one, turns out to be the most treatable crisis there is.

That is where we are going, week by week. Not to make you cope. To help you see.

Next week: why a full night’s sleep stopped fixing it.

Not because I had stopped loving cardiology. I loved it still. I resigned because I could no longer think properly inside it, and a cardiologist who cannot think clearly is a hazard wearing a lanyard.

Nobody collapses in a heap. That is the part people misunderstand. You do not run out of intelligence. You run out of something quieter. Room. The half-second before you speak. The willingness to ask the awkward question on the ward round. The capacity to hold two possibilities open when everyone in the room wants you to close them down and move on.

I started calling that quiet thing oxygen, because it behaves like oxygen. You only notice it when it runs low, and by then your judgement is already slurring.

This publication is about Cognitive Oxygen® (COx): the conditions that wise action depends on, and what it takes to protect them when the pressure will not stop.

Here is what I have learned, first as a clinician and then as a coach. Three things go before anyone sees them go.

Judgement goes first. Tired leaders do not make obviously stupid decisions. They make subtly worse ones, faster, with more confidence. The confidence is the tell. Pressure pushes people towards certainty exactly when the situation is calling for clarity, and certainty under complexity is not strength. It is a smaller version of the truth, wearing a louder voice.

Humanity goes second. The person who used to listen starts interrupting. The leader who used to ask starts telling. Not because they became unkind, but because kindness is metabolically expensive and they have run out of margin to pay for it. I have watched good doctors become difficult to work with inside a single bad winter, and I have been one of them.

The truth goes last, and it goes silently. This is the dangerous one. When a leader runs low, the people around them quietly decide it is no longer safe to bring bad news. The corrections stop. The early warnings stop. The leader experiences this as calm and reads it as competence. It is neither. It is the sound of a team that has stopped telling you what you most need to hear.

None of this is a character flaw. That is the whole point. It is a predictable response to depleted conditions, and conditions can be protected. We protect oxygen for patients without a second thought. We have never thought to protect it for the people making the decisions.

So that is the work. Three commitments, said plainly. Protect judgement, so decisions stay sound when the stakes are high. Preserve humanity, so the team stays whole and stays honest. Generate usable clarity, so people can orient and act inside the fog rather than pretending the fog has lifted.

I will not pretend this is gentle. Some of what I write will sit uncomfortably with how leadership is rewarded in healthcare, where stamina is mistaken for strength and the leader who admits they are running low is read as the weak one. I think we have that exactly backwards. The leader who can see their own oxygen falling, and say so, is the safest person in the building.

I called the principle margin is medicine before I fully believed it. I believe it now. The margin you keep, the half-second you protect, the awkward truth you make it safe to say, these are not the soft edges of the job. They are the clinical core of it.

You did not run out of talent. You ran out of air.

Let me show you how to get it back.

It is Saturday on-call. By lunchtime, you have seen twelve admissions. The pace is steady, the pressure is familiar, and you handle it with the practiced ease of a consultant who has seen it all before. You move through the wards, making decisions, steadying the team, and managing the quiet chaos that defines a weekend in a busy hospital.

By the time you finally climb into bed at half past two in the morning, you have made dozens of decisions. Three of those decisions were not small. They were the kind of calls that change lives, the kind that require you to weigh complex variables in seconds. You have worked a twenty-hour day on a handful of snatched calories and a great deal of adrenaline.

You get four hours of sleep.

On Monday morning, you are in clinic. The first patient sitting in front of you is a woman with a complex history and a list of questions she has waited six months to ask. She deserves a fresh mind. She deserves a clinician who can hear the thing she is not quite saying.

She does not get one. Instead, she gets the residue of your weekend.

You are calm. You are fluent. You are entirely competent. To the patient, to your colleagues, and even to yourself, you look like a model of resilience. You are still standing. You are still working. But something is missing. There is a thinning of the air.

This is the part of resilience that nobody photographs.

The Margin for Thought

We are often told that we need more resilience. The word is thrown around as a solution to every systemic failure and every personal struggle. If you are tired, you need to be more resilient. If the system is breaking, the people within it must be more resilient.

I believe this is a fundamental misunderstanding of the problem.

Most healthcare leaders are not short of resilience. They are some of the most resilient people on the planet. They have survived decades of high-stakes environments, navigated constant change, and held onto their humanity in the face of immense suffering. They are not running out of the will to continue.

They are running out of Cognitive Oxygen® (COx).

Cognitive Oxygen® (COx) is the margin a leader needs to pause. It is the mental space required to notice what is happening, to question assumptions, to hold complexity, and to choose the next right move rather than simply reacting to the loudest noise.

When we talk about oxygen in a clinical sense, we understand that it is finite. We know that without it, cells begin to struggle and eventually die. We do not ask a patient with low oxygen levels to simply be more resilient. We recognise that they are in a state of physiological crisis.

The mind works in much the same way. Attention is not infinite. Judgement has a metabolic cost. Compassion has a bandwidth. Every decision you make, every conflict you navigate, and every hour of sleep you lose thins that margin.

The Silence of Cognitive Hypoxia

When physical oxygen levels drop, the body has ways of sounding the alarm. In the world of leadership, the signs of a thinning margin are much more subtle. I call this state cognitive hypoxia.

The danger of cognitive hypoxia is that it does not cause people to fail loudly. Instead, they deteriorate quietly. Because you are a high-performing professional, you have spent years learning how to mask fatigue and stress. You know how to sound certain even when you are unsure. You know how to maintain the appearance of authority while your internal capacity is depleted.

This deterioration shows up across four linked domains:

Cognitive clarity. Your ability to process information slows. You find it harder to separate the signal from the noise. You start to rely more on shortcuts and heuristics, which are useful until they lead you into an unforced error.

Emotional steadiness. Your fuse becomes shorter. You might find yourself becoming more defensive or less empathetic. The emotional labour of leadership, which usually feels manageable, starts to feel like an impossible weight.

Behavioural discipline. You stop doing the things you know are right. You skip the team debrief. You avoid the difficult conversation. You stop questioning the status quo because you simply do not have the energy to deal with the answer.

Physical recovery. Your body stops bouncing back. The four hours of sleep no longer feel like enough to get you through the day. The fatigue becomes a baseline rather than a temporary state.

Because you stay calm enough and fluent enough, nobody sees this deterioration. The system continues to demand more, and because you are resilient, you continue to give it. But the quality of your thinking and the depth of your leadership are being slowly starved of the margin they need to thrive.

The Three Layers of Capacity

Restoring Cognitive Oxygen® (COx) is not as simple as taking a holiday or attending a wellness seminar. It requires a fundamental shift in how we view our capacity and our responsibility to the teams we lead. We have to look at the three layers where this margin is created or destroyed.

First, there is the inner layer. This is your own capacity to notice and regulate your internal state. It is the ability to recognise when your margin is thinning and to have the tools to pause and reset. It is about understanding that your brain is a biological organ with physical limits.

Second, there is the social layer. This is the team’s ability to speak, listen, repair, and trust. A team that lacks psychological safety is a team that consumes vast amounts of cognitive oxygen just to navigate internal politics and fear. A healthy social layer acts as a shared lung, allowing the team to think more clearly together than any individual could alone.

Third, there is the structural layer. This is the design of the system itself. It includes the governance, the meeting schedules, the time for recovery, and the windows of quiet that are built into the work. If the structure is designed to consume every second of your time, no amount of personal resilience will save you from cognitive hypoxia.

Why This Matters Now

I am starting this Substack because I believe the current pace of healthcare leadership is unsustainable. We are losing too many good people not because they aren’t strong enough, but because they are being asked to lead in an environment that is cognitively suffocating.

My goal is to explore how we can reclaim that margin. We will look at how clinicians actually think and decide, and how we can use tools like Red Team Thinking® to challenge our assumptions and reduce unforced errors. We will talk about the human cost of the relentless pace and how we can build systems that respect our carrying capacity.

This is a space for leaders navigating the reality of clinical work, not a home for generic corporate advice. It is a space for leaders who want to lead with calm authority and compassion, even when the stakes are high and the certainty is low.

Oxygen First

I have spent almost 25 years in high-stakes clinical leadership across seven countries. I have been the consultant on that Saturday on-call and have felt the air thinning in my own leadership. As a cardiologist and executive coach, I founded the School of Cognitive Oxygen® to help leaders create the cognitive margin needed to think clearly, decide wisely, and remain deeply human under pressure. Together with my co-author, , we explore what it takes to protect that margin—not only for leaders, but for the teams and systems they serve.

I invite you to subscribe and join this conversation. We will look at the tools of metacognition, the practices of emotional intelligence, and the structural changes that can restore the oxygen to our systems.

We cannot change the pressure of the work, but we can change how we respond to it. We can choose to stop relying solely on resilience and start protecting our capacity to think.

Dr Toomas Särev FRCP

Consultant Cardiologist & Executive Coach

Creator of the Cognitive Oxygen® (COx) framework