Nap time!

Photography by Gregory Pappas

Shail Bhatt explores the neuroscience behind every student’s favourite activity, sleep.

I’m sure most of us can agree that a good night’s sleep is one of the best things in life. The importance of sleep on our health, on our daily routines, and the way we live our lives is pivotal. There are many processes that control wakefulness and sleep, and their interplay is intricate and necessary. So, how exactly do we fall asleep?

The most stereotypical characteristic associated with sleepiness is yawning: one of the most fundamental and inherent actions of the body. Yawning isn’t learnt, and it’s something that sticks with us throughout our life; we begin yawning before we’re born, and images have even been taken of a foetus yawning in the first trimester. While yawning and sleep aren’t mutually exclusive events, it has been seen that yawns are more frequent when one is sleepy (you’re yawning now, right?). It was originally thought that this process occurs in response to oxygen deprivation, but a recent study titled Why do we yawn? has hypothesized that it allows us to transition from one behavioural state to another: from sleeping to awake, from tired to alert, from bored to attentive, from anxious to calm. This is why we yawn when we’re stressed, or when we’re getting bored in class. It has also been noticed that yawning brings cool air into the body and prevents our brains from overheating, and this promotes wakefulness and attentiveness. 

But what causes us to feel sleepy, and when do we decide to lie down and close our eyes? There are many hormones, neurotransmitters (chemicals that allow signalling between nerves), environmental conditions, and bodily mechanisms that control our sleep-wake schedule. For example, the amount of light is a crucial regulator of sleep. When it gets darker, receptors in our eyes pick up this change and send signals to the suprachiasmatic nucleus, the SCN, which is a part of the brain that controls the release of sleep-regulating hormones. As night falls, another part of the brain, the pineal gland, also gets activated, and releases a hormone called melatonin. This regulates our body temperature and sleep-wake cycles, often numbing sensitivity to stimuli in order to facilitate sleep. 

The circadian rhythm is our body’s internal clock: it controls how we sleep and when we wake up, following a particular pattern. Controlled by the SCN, sleep is normally deepest between 2-4 AM, although changes to the circadian rhythm occur throughout our life cycle. These are most influential during adolescence, where a ‘sleep-phase delay’ may occur, distorting the usual circadian rhythm. In teens, melatonin is actually produced 3-4 hours later, which keeps them up late and leaves them feeling drowsy in the morning, as they’re still producing melatonin when they wake up. 

Apart from the circadian clock, there is another process, known as the sleep-wake homeostat, which keeps track of the need for sleep. This homeostat regulates our sleep drive, which gets stronger for every hour one stays awake beyond the circadian clock, and extends the subsequent sleep. 

There are many different stages of sleep, but these can be classified as one of two distinct types: REM (rapid-eye-movement) and non-REM sleep. Stage 1 non-REM sleep is the point where the body transitions from awake to asleep, slowing the heart rate and lowering the depth of breathing. Stage 2 non-REM sleep is light sleep and is the most common sleep stage. During this phase, the muscles relax, body temperature drops, and brain activity is lessened. Stage 3 non-REM sleep is deep sleep, during which the heart rate and depth of breathing are at their slowest. The final – and perhaps most fascinating – stage is REM sleep, characterised, as the name suggests, by random and rapid movements of the eyes. This is the phase where dreaming and memory formation occur, and breathing levels increase; although muscle activity is inhibited, the body behaves as though it is awake. 

When the sun rises and the day begins, the SCN raises the body temperature and releases specific chemicals like cortisol which cause us to awaken. Histamine is an important neurotransmitter here, considered to be the ‘master’ promoter of wakefulness (this is also why antihistamines often cause drowsiness). Other neurotransmitters like serotonin and acetylcholine also promote alertness and encourage wakefulness. A unique chemical involved in the sleep cycle is orexin, which, though produced by only very few neurons, triggers waking up. Deficiencies in this chemical can cause sleep disorders like narcolepsy.

The science of sleep, transitioning from dusk till dawn, and from night to day, is an intricate interaction of the body’s processes and involves neurotransmitters, hormones, and various neural circuits. If sleep is disrupted, it can have negative consequences on your temperament and can even put you at risk of diabetes, heart disease, and obesity. So, to those people who stay up till 4 AM watching Netflix or writing assignments last-minute, get your sleep in check!