Every morning, something extraordinary happens.

Every night, your brain enters one of the most complex physiological states known in biology.

Every morning, it has to leave it.

This sounds obvious.

It isn't.

Modern sleep science has spent decades studying how we fall asleep. Researchers have mapped sleep stages, identified REM and non-REM cycles, described circadian rhythms, and investigated everything from insomnia to dreams.

Yet there is a striking imbalance in the scientific literature.

The transition into sleep has received far more attention than the transition out of sleep.

A 2019 review in Nature and Science of Sleep notes that the sleep-to-wake transition has received "comparatively little attention" despite its importance for cognition, safety and performance. A more recent 2024 neurophysiology review reaches a similar conclusion, arguing that awakening remains difficult to characterise because it is not a simple switch but a rapidly changing transitional brain state.

That is surprising.

Because every human being experiences this transition every single day.

We Think Waking Up Is an Event.

Neuroscience Suggests It Is a Process.

If someone asked you,

"When do you wake up?"

you would probably answer,

"When I open my eyes."

From the perspective of neuroscience, that answer is incomplete.

Researchers increasingly describe awakening as a progressive recovery of brain function, not a binary event.

Sleep and wakefulness are often spoken about as though they are opposite states separated by a clean boundary.

The evidence suggests otherwise.

Instead of a light switch,

awakening appears more like a sunrise.

Different neural systems recover at different speeds.

Some become active almost immediately.

Others require considerably more time.

This means that consciousness may return before full attention.

Movement may return before planning.

Awareness may return before judgment.

In other words,

being awake is not the same thing as having a fully awake brain.

This single idea explains a remarkable number of everyday experiences.

Why you can silence an alarm but forget doing it.

Why conversations feel unusually difficult first thing in the morning.

Why emails written immediately after waking sometimes seem surprisingly poor when read later.

Why driving or making important decisions immediately after waking can be riskier than most people realise.

The problem may not be motivation.

It may simply be that your brain is still completing one of the most complex transitions it performs every day.

The Discovery of Sleep Inertia

The scientific term for this transitional state is sleep inertia.

The phrase first appeared in the sleep literature decades ago to describe something that almost everyone has experienced but few people had considered scientifically:

Why does someone who is technically awake still feel partially asleep?

Researchers define sleep inertia as a temporary period of reduced alertness, impaired cognitive performance and a desire to return to sleep immediately after awakening.

Importantly,

sleep inertia is not considered a disease.

It is regarded as a normal physiological phenomenon.

The fact that it happens after a full night's sleep surprised early researchers.

One might assume that eight hours of restorative sleep should leave the brain instantly refreshed.

Instead, experiments repeatedly demonstrated measurable declines in vigilance, reaction time, memory and decision-making immediately after waking—even following a normal night's sleep.

That observation forced scientists to reconsider a long-held assumption.

Perhaps waking up was never instantaneous in the first place.

A Brain That Is Both Asleep and Awake

One of the most fascinating developments has come from modern neuroimaging.

Using EEG, intracranial recordings, PET imaging and functional MRI, researchers have found evidence that the brain immediately after awakening may not exist in a purely "awake" state.

Instead,

different regions appear to occupy different functional states at the same time.

The 2024 review in Neurophysiologie Clinique describes sleep inertia as being underpinned by a heterogeneous cerebral state, where patterns associated with local sleep coexist alongside patterns associated with local wakefulness. Rather than every neural network switching simultaneously, the emerging evidence suggests that awakening is spatially uneven across the brain.

This is a profound shift in how we think about mornings.

For decades, sleep and wakefulness were treated as mutually exclusive categories.

Now researchers increasingly discuss the possibility that the brain briefly occupies a mixed state.

You are not fully asleep.

You are not fully awake.

You are temporarily both.

We Think We Wake Up in an Instant.

Science suggests we don't.

Most of us imagine waking as a simple event.

The alarm rings.

Our eyes open.

Sleep ends.

Wakefulness begins.

From our subjective experience, the transition feels almost instantaneous.

Yet decades of sleep research suggest that this intuition is misleading.

The first minutes after awakening represent a distinct neurophysiological state with its own behavioural characteristics, its own brain activity and its own cognitive limitations. Sleep researchers have a name for this state: sleep inertia. Far from being a sign that something is wrong, sleep inertia appears to be a normal part of human biology, although its intensity varies greatly between individuals.

Understanding this changes the question.

Instead of asking:

"Why am I still sleepy?"

we might ask:

"What exactly is my brain doing while it wakes up?"

That question turns out to be far more interesting.

The Forgotten Half of Sleep Science

Over the past seventy years, sleep science has transformed our understanding of sleep.

Researchers have identified REM and non-REM sleep, mapped circadian rhythms, described sleep architecture, uncovered the role of adenosine and melatonin, and linked sleep to memory, immunity, metabolism and emotional regulation.

But there is an imbalance in this body of knowledge.

Much of sleep science focuses on entering sleep or remaining asleep.

The transition out of sleep has received comparatively less attention, despite occurring every day and despite having measurable effects on cognition, mood and performance. Recent reviews explicitly identify the sleep-to-wake transition as an area requiring further investigation because conventional sleep staging does not adequately capture the rapidly changing, spatially heterogeneous brain activity during awakening.

This gap matters.

Because for most people, the value of a night's sleep is not judged while they are asleep.

It is judged by how effectively they function after waking.

Waking Is Not a Switch. It Is a Reconstruction.

One of the most important shifts in modern neuroscience is the move away from thinking of sleep and wakefulness as two sharply separated states.

Instead, awakening is increasingly viewed as a dynamic reconstruction of brain function.

Imagine reopening a large international airport after an overnight closure.

The runway lights come on first.

Ground crews begin moving.

Security systems restart.

Aircraft begin taxiing.

Passengers enter the terminal.

Eventually, the airport returns to full capacity.

No one would describe that as a single moment.

The brain appears to behave in a similar way.

Neuroimaging studies using EEG, intracranial recordings, PET and functional MRI suggest that different neural networks recover at different rates after awakening. Rather than switching simultaneously, brain regions show a coexistence of wake-like and sleep-like activity for a period after waking.

This is one of the most intriguing findings in the recent literature.

Immediately after waking, the brain may not be entirely awake.

Nor is it entirely asleep.

It temporarily occupies a hybrid state.

Sleep Inertia: More Than Morning Grogginess

Most people describe sleep inertia as feeling "groggy."

Scientists define it much more precisely.

Sleep inertia is a transient period of reduced alertness accompanied by measurable impairments in attention, reaction time, working memory, decision-making and subjective alertness. These effects are reproducible in laboratory studies and have been demonstrated using psychomotor vigilance tests, arithmetic tasks, memory assessments and brain imaging.

Importantly, sleep inertia is not merely a feeling.

It is a measurable state.

In controlled experiments, participants consistently perform worse immediately after awakening than they do later in the same morning.

Performance gradually improves as the brain completes its transition into stable wakefulness.

This distinction matters.

Feeling sleepy is subjective.

Reduced cognitive performance is objective.

The two often occur together.

The Brain Does Not Recover Uniformly

One reason sleep inertia has fascinated neuroscientists is that not every mental ability returns at the same speed.

Research suggests that some functions recover relatively quickly, while others recover much more slowly.

Studies indicate that executive functions—planning, judgement, flexible thinking and complex decision-making—may remain impaired after more basic functions have already recovered. Functional imaging has shown lingering alterations in brain connectivity and persistent sleep-like signatures during this period.

This helps explain a common experience.

You may be capable of walking to the kitchen and making coffee.

Yet you may struggle to:

• solve a difficult problem,
  
• remember what you intended to do,
  
• make an important decision,
  
• or write your best work.
  

The limitation is not necessarily motivation.

It may simply reflect the order in which different brain systems recover.