Sleepiness has a build-up.
Sleep pressure is the drive to sleep that grows while you are awake and eases during sleep. It works alongside your body clock.
Follow the pressure through a day.
Choose a moment. This is a simple illustration, not a measurement of your sleepiness or a prediction of when you will fall asleep.
Read the pattern
Sleep pressure usually falls with sleep, builds during wakefulness and can be reduced by a nap. How sleepy you feel also depends on your body clock, prior sleep and other factors.
Adenosine is part of the picture.
Adenosine is one chemical involved in the growing drive to sleep. Its signalling increases during wakefulness and reduces during sleep. Sleep pressure is a biological process, not a simple battery percentage.
A coffee makes you feel more awake after a short night. Has it replaced the missing sleep?
Read the explanation
Caffeine blocks some adenosine signalling. It can help you feel alert for a while, but it does not repay missed sleep or remove your need for it.
Why a late nap can change the evening.
A long or late nap can lower the drive to sleep before bedtime. It may then be harder to fall asleep, even though the clock says it is bedtime.
Naps can also be useful in some circumstances. If you struggle with night-time sleep, notice whether naps and late caffeine are affecting your pattern. Persistent excessive sleepiness needs assessment; do not drive when sleepy.
The clock
“Is this my usual biological time for sleep?”
The pressure
“How much drive to sleep has built up?”