How your body controls the feelings of sleepiness and alertness during the day and night.
Many people notice that they naturally experience different levels of sleepiness throughout the day but what causes these patterns?
Sleep is regulated by two body systems – sleep/wake homeostasis and the circadian biological clock.
When we have been awake for a long period of time, sleep/wake homeostasis tells us that it is time to sleep.
It also helps us maintain enough sleep throughout the night to make up for the hours of being awake.
Our internal circadian biological clocks, on the other hand, regulate the timing of periods of sleepiness and wakefulness throughout the day.
The circadian rhythm dips and rises at different times of the day. Adults’ strongest sleep drive generally occurs between 2-4am and in the afternoon between 1-3pm, although there is some variation depending on whether you are a “morning person” or “evening person”.
The sleepiness we experience during these circadian dips will be less intense if we have had sufficient sleep, and more intense when we are sleep deprived.
The circadian rhythm also causes us to feel more alert at certain points of the day, even if we have been awake for hours and our sleep/wake restorative process would otherwise make us feel more sleepy.
Changes to this circadian rhythm occur during adolescence, when most teens experience a sleep phase delay. This shift in teens’ circadian rhythm causes them to naturally feel alert later at night, making it difficult for them to fall asleep before 11pm.
The circadian biological clock is controlled by a part of the brain called the Suprachiasmatic Nucleus (SCN), a group of cells in the hypothalamus that respond to light and dark signals.
From the optic nerve of the eye, light travels to the SCN, signaling the internal clock that it is time to be awake. The SCN signals to other parts of the brain that control hormones, body temperature and other functions that play a role in making us feel sleepy or awake.
In the mornings, with exposure to light, the SCN sends signals to raise body temperature and produce hormones like cortisol. The SCN also responds to light by delaying the release of other hormones like melatonin, which is associated with sleep onset and is produced when the eyes signal to the SCN that it is dark. Melatonin levels rise in the evening and stay elevated throughout the night, promoting sleep.
Circadian disruptions such as jet lag put us in conflict with our natural sleep patterns, since the shift in time forces the body to alter its normal pattern to adjust.
This is why jet lag can leave travellers feeling poorly and having more difficulty thinking and performing well. But these symptoms can also occur in everyday life, when the circadian rhythm is disrupted by keeping long and irregular hours. Because of this, it is important to keep a regular sleep schedule and allow plenty of time for quality sleep.