Our body has an internal drive towards maintaining an internal balance by modifying physiological processes. This natural tendency for equilibrium does not only influence sleep, it also affects our need for food, water, etc.
For example, if we have not eaten anything for a while, we get hungry. When we feel hungry, we eat and we are not longer hungry. The same thing happens with thirst and also with sleep.
Our internal drive for sleep equilibrium takes into account two basic aspects:
The time that has passed since we last woke up,
Our sleep debt, which is cumulative.
We have experienced, if we have a normal sleep cycle, and we should have seen in our children how we get sleepy when we stay awake longer than usual. This sleepiness increases as time passes and we are still awake, and it decreases as we get the sleep we need.
Our body needs a certain amount of sleep to function effectively, it usually varies around eight hours. When we get less than the eight hours (approx.) of sleep we need, we start to accumulate a sleep debt, just as with money when we spend more than what we earn having no previous savings.
Let’s picture a boy who needs 9 hours of sleep every day, but he had to stay up tonight because he had a party so he just sleeps 6 hours, and yesterday he stayed up doing homework and slept 7 hours; in addition to these two nights with less sleep, tomorrow he’s going to an event in which his mother will be honored, so he will have to attend and he will only get 5 hours of sleep. In just three nights he will accumulate 9-6=3 hours tonight, 9-7=2 hours the day before, and 9-5=4 hours tomorrow, adding 9 hours of sleep debt (3+2+4). This sleep debt will make him very sleepy, this sleepiness forces him to sleep, as it is is his own body’s demand to get the 9 hours of sleep per night he needs.
When and why we fall asleep is determined by different factors working together like the need of sleep time our body requires for optimal functioning; paying our sleep debt; our circadian clock (discussed in last week’s post); and our will.
Depending on the time of the day and our sleep debt, our internal drive for sleep and our circadian clock can work together to promote sleep-at night; or they can work against each other when one causes sleepiness and the other expects us to be awake-during the day…