If you’ve noticed that you’re feeling sleepier at certain times of day and more energetic at other times, two of your body systems are to blame. Namely, your circadian rhythm and the sleep/wake homeostasis regulate your sleep drive, i.e. when your body needs sleep.

To be more precise, the sleep/wake homeostasis is responsible for balancing the need for sleep, i.e. the sleep drive, also known as sleep pressure, with the need for wakefulness. So, when you’re awake for a longer period of time, your sleep drive signals that it’s time to sleep. Then, once you get a decent amount of sleep, your sleep drive decreases, and the need for alertness increases, signaling that it’s time to wake up.

What’s more, if sleep/wake homeostasis was the only system in charge of regulating our sleep drive, we would likely be most alert in the morning and most sleepy at night. However, this system works together with the circadian rhythm to regulate sleep schedule which enables us to have the same level of alertness at 9:00 a.m. and 5:00 p.m.

Sleep Drive and Circadian Rhythm

Our alertness levels decrease and increase throughout a 24-hour period in accordance with certain environmental cues such as sunlight. In general, people tend to feel most tired right after midnight and after lunchtime. But, in the case of sleep deprivation, tiredness feels even more intense. On the other hand, we feel less tired when we’ve had a good night’s sleep.

In addition, as we already mentioned, sunlight affects circadian rhythm and can hamper our internal body clock in case of exposure to artificial light after daytime. In turn, our sleep drive gets affected too.

But, how does circadian rhythm work? Namely, the Suprachiasmatic Nucleus (SCN), a part of the brain responding to light and dark signals, controls circadian rhythm. So, when eyes look at a light, the retinas immediately signal the SCN which as a result triggers many bodily reactions. Such reactions include cortisol hormone production, increased body temperature, appetite, sleep drive, etc.

On the other hand, when it’s dark outside, the eyes send signals to the SCN which then increase melatonin levels, a hormone promoting sleep, and decrease body temperature. And, as long as the eyes perceive light, even artificial light from electronic devices or indoor light, the SCN suppresses the production of melatonin making it difficult to fall asleep.

A Thrown-Off Sleep Drive

Although it sounds surprising, some people have their circadian rhythm thrown off. When this happens, daytime tiredness and feeling wired at night increase. Any changes in sunlight exposure can result in insomnia and daytime sleepiness. This effect is most noticeable in jet lag, when the circadian rhythm gets different time and light cues and forced the body to adjust.

Similarly, when working irregular hours or overnight shifts, the circadian rhythm can also get disrupted. Indeed, shift workers are at higher risk of developing insomnia, suffer from extreme daytime sleepiness, mood issues, and having a workplace accident. Plus, these workers are also likely to have their hormonal balances, especially cortisol, melatonin, and testosterone, disrupted.

Luckily, even though it takes time and effort, you can actually change your circadian rhythm and adjust your sleep drive. You can do so by following a strict sleeping routine, i.e. go to sleep and wake up at the same time every day of the week. Still, ensure that you get at least 7 hours of sleep each night.