Why can your body have a “sleep debt” but not a “sleep surplus”?


Why does my 15 hours of sleep on the weekend not counteract the 4 hours I get on a weeknight?

In: Biology

Not getting enough sleep doesn’t let the body and brain finish its cycles. The damage and lack of sleep build up (debt)

Once the body is finished with the cycles, that’s it. It can’t store sleep to repair future damage and build future neural pathways. It can only work on what has already been done.

Its like your laptop – once you use is for a while you need to charge it up and if it falls to 0 you need to charge it before it works again but if you leave it overnight the max is only 100%

While your brain works, it slowly creates byproducts by standard biological processes, like most of your body, and these byproducts need to be removed from the system. Throughout the day, your brain slowly swells as it works, and this swelling reduces how efficiently your body can “clean away” the byproducts in your brain. You need to sleep so your body can balance the cleaning rate with the production rate of the byproducts. The swelling also goes away when you sleep, as the brain is used less.

When you don’t sleep, the swelling doesn’t go away, and continues to increase, making you even worse at maintaining the brain. This is why eventually you can die from lack of sleep.

When you wake up well rested, your brain is as “not-swollen” as it can be, and as “clean” as it can be. You can’t get more ready for stuff, and you can’t clean away byproducts that aren’t there yet!

Your brain is like a trash can. Throughout the day it fills up with garbage, and at night that garbage is slowly removed. If you don’t get enough sleep, only some of the garbage gets taken out. Over time, it builds up until your can is almost always full. A full can is bad and makes you fall asleep or hallucinate.

However if you sleep lots and lots, your trash-can can’t get under 0% full. It will always only hold a certain amount of garbage.

Same like you can’t pee when there is nothing in your bladder, you don’t need sleep before our body has a need for it

You *can* run a sleep surplus, but it doesn’t last as long as running sleep debt does. Sleep is complicated; we don’t understand fully how it works.


Since its winter and I am thinking about snow, another ELI5 comparison is how we get rid of the snow.

If we don’t plow or anything the snow can build up to many feet high, but we can’t plow in advance. The road can only be clear at best, but snow can keep building up.

Sleep is the body doing a kind of maintenance, it can only get the body to a clear and ready state.

The harmful effects of staying awake can keep building up until it’s cleared properly.


FYI It’s not exactly true.

You can store up a very limited sleep surplus : it’s called “ prophylactic napping”. It’s essentially done right before an all-nighter. The benefit disappear if you wait. Also it will cover for one night at best.


Other people have given great answers, but to add – sleep debt is not a very accurate name for chronically under-sleeping.

Sleep debt is more like cumulative damage to your well-being, some of which can be repaired by healthy sleep habits.

It’s really important that people know that sleep debt isn’t something you can repay 1:1, you can’t constantly get 4-5 hours of sleep and “catch up” on the weekend and be square.

There was a guy who took this to the extreme in the 90’s, inspired by a radio contest. He stayed awake for 11 days, setting a record. When he finally did sleep, if memory serves, he slept like 10 hours? He developed permanent brain damage, antisocial tendencies, and permanent severe insomnia. (And honestly, he’s lucky to be alive.)

The moral of the story, 5 hours on Thursday night and 11 hours on Friday night does not equal 8 and 8. Be careful out there, folks.

Your brain is like a bunch of young kids cooped up in a house. Throughout the day, the kids are running around making messes, and the dishes and laundry they use keep piling up. The adult(s) can’t take care of the mess until the kids aren’t needing constant attention. You can fall behind on tidying and cleaning but even if you have extra time, you can’t clean up the next day’s messes before they happen.

The brain is similar. While it’s awake, it builds up “messiness” that it can’t take care of until it’s asleep, when the body and brain slow down enough to focus on cleaning up the mess.

You can bank sleep according to some studies. Military and shift workers who may need to work through sleep cycles can use this to improve mental and physical capabilities during extended wakefulness. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4667377/