Sleep pressure: the overflow that makes us sleepy

In terms of sleep balance, we most often hear of circadian rhythms being reset each day by synchronizers, one of the best-known of which is alternating light and darkness. But what we don’t hear as much about, and yet is of vital importance, is sleep pressure.

A quick reminder of circadian factors

Circadian regulation of sleep exists even without external temporal cues. But this rhythm is slightly out of sync with the 24-hour day-night rhythm. So, to resynchronize it each day, we need external synchronizers, also known as zeitgebers, meaning “time givers”, such as mealtimes, exercise, and of course light, one of the most powerful. If you were to experience life without any external synchronizers, you’d find that within a few days you’d be back in reality with a time difference from how you felt. This was the experience of speleologist Jean-Pierre Mairetet, who isolated himself in an abyss for 174 days, believing when he came out that he had only spent 86 days there. And we know that this rhythm has key moments during the day when it’s hard to resist sleep and harder to fall asleep.

Sleep pressure or the irresistible need to sleep

Another little-known sleep-regulating factor that scientists have identified is sleep pressure. During our waking phases, we accumulate hypnogenic substances. This word, which comes from the ancient Greek hýpnos, “sleep” and gígnomai, “to generate”, simply means sleep generator. Above a certain quantity of these substances, whether naturally synthesized by the body or absorbed, sleep becomes unavoidable. Then, during sleep, they dissipate through mechanisms we’re beginning to learn about, and which we discuss in our article “Sleep, a matter of survival?” Dr K Espa Cervena, somnologist at the Cenas sleep center, likes to use the image of the bathtub to schematize this phenomenon: “When you’re awake, the tap is open and substances accumulate in the bathtub. This accumulation pushes you to open the stopper to evacuate the substances and avoid overflowing. When you go to sleep, you open this stopper.”

Sleepiness due to lack of sleep: an overflowing story

As Dr Espa Cervena describes it, if we think of sleep pressure as the story of a bathtub that fills up, then we can well imagine that if that bathtub fills up indefinitely without ever being emptied, then we’re going to be sleepy, and it will become increasingly urgent to sleep. But when the bathtub is really too full, it can be complicated and time-consuming to empty it completely. Now imagine that to empty your bathtub completely after a typical day, you need 8 hours. But your lifestyle, or sometimes your self-imposed rhythm, doesn’t allow you to sleep for 8 hours. When you wake up the next morning, you recap the bathtub without having completely emptied it. Then you fill it up again. At first, you won’t notice a thing, but after just a few days of this treatment, you’ll start to run out of filling space before it overflows, and you’ll feel the effects: drowsiness, reduced alertness and performance, irritability…. Taking the time to sleep, and sleep well, is therefore an effective way of gaining time when you’re awake.