You’re sitting at your desk, eyes heavy, struggling to focus on your screen. The words blur, your head droops, and suddenly, you jolt awake with a start. Or maybe you’re driving home after a long day, and for a few terrifying moments, you realise you don’t remember the last mile. This isn’t full sleep, but it’s not full wakefulness either.
Drowsiness is a strange and fascinating neurological state, a limbo where parts of your brain shut down while others stay online. It’s more than just being "a little tired"—it’s a complex and poorly understood phase of consciousness that plays a crucial role in how we function. Neuroscience suggests that when we are drowsy, our brain isn’t just slowing down—it’s literally partly asleep.
But what does that mean? Can different parts of the brain sleep independently? And if so, what are the implications for our daily lives, our safety, and even our long-term brain health? Let’s explore what science has uncovered about this bizarre halfway state between wakefulness and sleep.
Micro-Sleeps and the Fragmented Brain: What Happens When We’re Drowsy?
When we think of sleep, we usually imagine an all-or-nothing state: either we’re awake and aware, or we’re unconscious and dreaming. But in reality, sleep doesn’t always happen in a neat, predictable way. The brain doesn’t switch off all at once—it does so in patches.
Researchers have discovered that during extreme fatigue, individual neurons in the brain can enter sleep-like states even while the rest of the brain remains active. This phenomenon, known as local sleep, means that different parts of your brain might be in different states at the same time.
This helps explain why micro-sleeps occur—those brief, involuntary lapses in attention where your brain temporarily "blacks out" for a second or two. These micro-sleeps can happen while driving, working, or even in the middle of a conversation. The scary part? You often don’t realise it’s happening.
EEG studies show that during drowsiness, brain waves begin to mimic the early stages of sleep. Some neurons slow their firing rate, making it harder to concentrate, process information, or react quickly. This is why, when drowsy, you might experience:
Slow reaction times (making driving or operating machinery incredibly dangerous)
Impaired memory (because your brain is struggling to encode and retrieve information)
Hallucination-like distortions (such as feeling like time is stretching or objects are moving when they aren’t)
This state of partial sleep might be the brain’s way of forcing rest in conditions where full sleep isn’t possible. But while this might be useful from an evolutionary perspective—allowing our ancestors to stay semi-alert while resting—it’s a serious liability in modern life, especially when it comes to safety-critical tasks.
Your Brain on Drowsiness: The Pre-Sleep State of Confusion
Drowsiness isn’t just about feeling sleepy—it’s an actual altered state of consciousness. In this phase, brain activity shifts dramatically, creating some strange psychological effects.
One of the key features of drowsiness is cognitive instability. The brain struggles to maintain consistent levels of attention, often slipping between wakefulness and sleep without warning. This instability affects:
Memory Formation – Information learned in a drowsy state is often forgotten because the brain isn’t encoding memories properly. This is why students who try to study while sleep-deprived often struggle to recall what they read.
Perception – Sensory input becomes unreliable. Some people experience mild hypnagogic hallucinations—fleeting images, sounds, or sensations that feel dreamlike.
Time Distortion – Ever noticed how time seems to stretch and contract when you’re fighting to stay awake? This is because drowsy brains struggle to track time accurately.
Interestingly, drowsiness can mimic the effects of alcohol. Studies show that being awake for 18–24 hours straight can impair cognitive function as much as being over the legal alcohol limit. This is why sleep-deprived driving is as dangerous as drunk driving, yet far less stigmatised.
Why Some Parts of the Brain Sleep While Others Stay Awake
If you’ve ever seen a dolphin sleep, you might know that their brains do something remarkable: they sleep one hemisphere at a time. This is called unihemispheric sleep, allowing them to rest while still staying semi-alert to dangers.
Humans don’t have full unihemispheric sleep, but research suggests that we experience a mild version of it during drowsiness. Certain regions of the brain—particularly in the prefrontal cortex—begin to slow down before others, affecting our ability to concentrate and make decisions.
The thalamus, a crucial relay station for sensory information, also becomes less active, which explains why drowsy people are less aware of their surroundings. Meanwhile, the brainstem (which controls basic life functions like breathing and heart rate) stays fully operational.
This uneven shut-down process means that some parts of the brain are functioning normally while others are, quite literally, asleep. This is why drowsy people can still walk, talk, and perform automatic actions while being mentally "offline."
This raises an important question: can we train our brains to resist drowsiness? The answer is not really. Once drowsiness sets in, cognitive impairment is inevitable, no matter how much willpower you apply.
How to Combat Drowsiness: The Only Real Solution
If drowsiness is essentially a partial brain shutdown, what can we do about it? Many people rely on caffeine, loud music, or fresh air to fight it off—but these are temporary fixes at best.
Here’s what the science says about staying alert:
Short naps (10–20 minutes) are highly effective. Unlike micro-sleeps, which happen involuntarily, planned naps allow the brain to restore function safely. Even a brief nap can reset drowsy neurons and improve cognitive performance.
Bright light exposure can help signal the brain that it’s still daytime, delaying the onset of drowsiness. This is why shift workers and night drivers often benefit from blue-light therapy.
Physical movement (such as stretching or a brisk walk) temporarily boosts circulation and brain activity, but the effect wears off quickly.
Caffeine blocks the effects of adenosine, a neurotransmitter that promotes sleepiness. However, caffeine only delays drowsiness—it doesn’t eliminate it.
Prioritising sleep is the best long-term solution. Consistently getting 7–9 hours of sleep per night is the only way to truly prevent drowsiness-related cognitive decline.
What doesn’t work? Sheer willpower. Many people believe they can "push through" fatigue, but research shows that drowsy brains don’t function properly, no matter how much motivation you have.
Why Understanding Drowsiness Matters
Drowsiness isn’t just "feeling tired"—it’s a serious neurological event where parts of the brain literally enter sleep mode while the rest struggles to stay awake. The result? Slower thinking, impaired decision-making, and increased risk of accidents.
Understanding how the brain transitions into sleep helps us appreciate just how crucial proper rest is for cognitive function and safety. Whether you’re a student pulling an all-nighter, a long-haul driver, or simply someone who battles mid-afternoon fatigue, recognising the signs of drowsiness and responding appropriately could make all the difference.
So the next time you catch yourself zoning out, struggling to keep your eyes open, or forgetting what you just read, don’t ignore it. Your brain isn’t just tired—it’s slipping into sleep, one neuron at a time.