How does sleep happen?

Sleep wake homeostasis

This scientific phrase expresses what most of us intuitively understand: the longer you're awake, the more you want to sleep. This is due to the body's self-regulating system, the homeostatic sleep drive, which increases the pressure to sleep depending on how long you've been awake. After a time of insufficient sleep, this same impulse induces you to sleep longer or more deeply.

Circadian rhythm 

Circadian rhythms, which are a part of your body's internal clock, last about 24 hours and are involved in a variety of biological activities, including sleep. The strongest influence on circadian rhythms is light exposure, which encourages alertness during the day and drowsiness at night. However, other physiological parameters also exhibit a rhythm with a 24-hour period, for example, our body temperature or the secretion of certain hormones fluctuate in a 24-hour rhythm.

The hypothalamus, thalamus, pineal gland, basal forebrain, midbrain, brain stem, amygdala, and cerebral cortex are all involved in these multiple processes. The biological intricacy of sleep is further demonstrated by the involvement of so many regions of the brain in awake and sleep, including the sleep stages.

Why do we need a circadian rhythm?

With the rotation of the earth, there are periodically recurring changes such as changes from brightness to darkness, warmer and colder temperatures - this is why circadian rhythms are not only found in humans, but also in animals and plants. The essential function of the circadian rhythm is to classify life processes in terms of time.

The main influence on the circadian rhythm is light, which affects the production of melatonin, the hormone that gives daily and seasonal time cues, which peaks during the night in the absence of light. Melatonin production slowly increases at the onset of darkness and stays elevated for the course of an approximately twelve-hour period.

Although the hormone melatonin does not participate in the generation of sleep itself, it increases the sleep pressure we feel (i.e. tiredness and need to sleep), and thus times when sleep is going to occur.

There is also a link between our circadian rhythm and nutrition which has a temporal structure to our everyday life - this is referred to as the principle of chrononutrition or time-restricted-eating (TRE). It's a type of intermittent fasting in which daily food intake is adjusted to the circadian rhythm and restricted to 4-12 hours each day. It does not demand calorie restriction, but it does necessitate a daily eating window that is constant.

Jet lag

Most people have had experience of jet lag at some point in their lives, especially after long haul flights where you may need a few days to adapt to a new sleeping pattern. When travelling time zones, our circadian rhythm takes longer to adapt to the new time zone. Consequently, our internal clock is somewhat desynchronised with the new environment. This can lead to daytime sleepiness, headaches, stomach problems or difficulties falling asleep.

What chemicals and hormones are involved in sleep?

The mechanics of sleep-wake homeostasis and the circadian alerting system require a variety of substances and hormones. Changing from wakefulness to sleep causes changes in thousands of neurons in the brain as well as a complicated communication system that causes particular bodily reactions.

There is still a lot we don't know about the complex processes that control sleep, but scientists have uncovered some compounds that appear to be crucial gears in the sleep engine.

Adenosine, a chemical, is thought to play a key role in sleep-wake balance. When we're awake, adenosine builds up in our bodies, which appears to increase sleep demand. Caffeine, on the other hand, blocks adenosine, which could explain why it enhances alertness in some people. Neurotransmitters are chemicals that send messages throughout the nervous system, causing certain cells to stimulate or inhibit. GABA, acetylcholine, orexin, and serotonin are examples of neurotransmitters that promote alertness or sleep.

Hormones are also important for signalling and controlling sleep-wake cycles.

One of the most well-known hormones connected to sleep is melatonin, which promotes sleep and is naturally created as light exposure diminishes. Adrenaline, cortisol, and norepinephrine are other essential sleep-related hormones. Sleep can also alter the synthesis of important hormones that govern appetite, such as growth hormone, leptin, and ghrelin, which can impact sleep-wake homeostasis and circadian rhythms. 

These chemicals and hormones may have various functions in different people depending on their heredity, which is why some sleep disorders can run in families. The chemical and hormonal signalling necessary for sleep may also be influenced by the environment and lifestyle factors.