Why do you get sleepier the longer you stay awake?
Your brain keeps a running tab on how long you have been up, and one chemical is part of the tally.
▶ Start the storyBecause a countdown has been running since you woke up. The longer you stay awake, the stronger the pull of sleep grows, and sleep researchers call that pull sleep pressure, or Process S. A neurotransmitter called adenosine is part of the story: its levels build up in the brain while you are awake, and higher levels correlate with feeling sleepier. During sleep it falls again, which is part of the sensation of restedness when you wake.
Process S is only half the story. The other half is the body clock, Process C, which counteracts the sleep drive during the day and adds to it at night. The two work together, and sometimes against each other: even a person with sleep debt can have trouble staying asleep at the peak of their circadian cycle.
Everybody who has pulled an all-nighter has met the result. The fatigue and the lost sleep are carried over to the following day, and subjectively humans seem to reach maximum sleepiness about 30 hours after waking. Caffeine fits the story neatly: its stimulating effect is credited mostly, though not entirely, to blocking adenosine receptors.
30 h
How exactly sleep debt works is still debated. Sleep debt does show some evidence of being cumulative, but scientists disagree on how much of it you can accumulate.
Quiz me
0/3
Recap
Process S counts hours awake, Process C tells the time of day, and sleep timing comes from both.
💡 A trick to remember it · S is for sand in the hourglass, C is for the clock on the wall: you need both to know when it is time to sleep.
Surprising fact · People seem to reach maximum sleepiness about 30 hours after waking.
Sources (4)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.