Trying to catch up on lost sleep? It may not help with sleep deprivation.

The sleep-deprived brain is not a smart brain. When we are short on shut-eye, our ability to process information, remember or sustain attention on tasks takes a hit, research consistently shows.

After one night of no sleep or multiple nights of restricted sleep, people perform worse on a standard reaction time experiment measuring vigilance. When prompted to press a button as soon as they receive a cue, sleep-deprived individuals are more likely to respond slower (or not at all) and to also erroneously hit the button with no cue.

And the more sleep deprived people become, the more lapses of attention they have. Crucially, these lapses of attention occur sporadically, so they might not even notice most of the time if there were consequences.

Unfortunately, many people are chronically sleep-deprived, losing out on hours of sleep during the workweek.

One of the ways many of us try to make up for lost sleep is by sleeping in when we can, over the weekend or days off. This can mitigate some of the ill effects of chronic sleep loss, research shows. However, sleeping more over a weekend of two (or occasional three) days is not long enough for most people to pay down the sleep debt incurred during the workweek. If we chronically lose sleep for five nights and then make up some of it for two nights, eventually, we will hit a suboptimal equilibrium.

“We all live in what is our personal normal,” said Hans Van Dongen, director of the Sleep and Performance Research Center at Washington State University. “But it isn’t our personal best.”

How sleep loss affects cognition and the brain

When we use certain neural pathways to perform intensive cognitive tasks — a reaction time experiment, for example — there is a cost. Neurons need energy and create metabolic waste as they process information.

“You need to resupply or you need to clean up,” Van Dongen said. “Or more likely probably both.”

When we step away from a task for a break, that portion of the brain gets a chance to rest and recover, and stops functioning properly, falling into what researchers call “local sleep.” When groups of neurons fall into local sleep, they show synchronized activity patterns typical of a sleeping brain even if the surrounding neurons and the rest of the brain remain awake.

The problems arise when we don’t take a break and continue to use these specific neural pathways past their limit. “They will just fall asleep on you while the rest of you is still awake, but they won’t function,” Van Dongen said. “And you have one of these lapses of attention.”

And the longer we stay awake, the more brain pathways will become affected and impaired. “If local sleep becomes all global … then you’re going to literally just fall asleep,” Van Dongen said.

Recovery sleep does not quite get us back to baseline

Experiments simulating weekday sleep restriction followed by weekend recovery sleep consistently show that while catching up on sleep helps, it does not bring people back to their previous baseline performance.

In a 2024 study, 52 healthy young adults were randomly assigned to one of three sleeping conditions.

One group slept for six hours during the five “weekdays” while another had a more variable short sleep schedule of either four, six or eight hours of sleep throughout the week. On the two “weekend” nights each group got eight hours of recovery sleep before returning to their restricted sleep schedule for another five “weekdays.”

A control group got eight hours of sleep throughout the study.

Subjects in both restricted sleep groups performed worse in cognitive testing compared with the control group. Even with the two nights of weekend recovery sleep, their performance continued to deteriorate, though slightly less in the variable short sleep group.

Other research has reported that even four straight nights of 12-hour recovery sleep did not completely restore cognitive deficits following chronic sleep loss.

“There’s no way you can cheat when it’s about sleep,” said June Lo, assistant professor at the Center for Sleep and Cognition at the National University of Singapore’s Yong Loo Lin School of Medicine and co-author of the 2024 study.

Our sleep history follows us

A reason one or two nights of extra sleep does not make up for many nights of reduced sleep is because our recent sleep history follows us.

In one 2023 study, researchers tested how one night of more or less sleep affects a pattern of sleep deprivation.

For five nights, 70 healthy adults slept for four hours. On the sixth night, they were randomized to sleep for varying hours, ranging from 0 to 12. They then continued with a second period of five nights with four hours of sleep.

Subjects who got fewer than four hours of sleep on the sixth night cognitively performed and felt worse, while those who got more than four hours of sleep performed better.

But unexpectedly, when the subjects returned to four hours of sleep, that night in between almost “didn’t really matter,” said Siobhan Banks, director of the Behavior-Brain-Body Research Center at the University of South Australia. The subjects’ cognitive performance continued to deteriorate along the same trajectory as if nothing happened.

“It’s because your baseline or your set point is changing over that time, because your body’s getting used to a different amount of sleep,” said Banks, who was a co-author of the study with Van Dongen. “If you’ve been sleeping really poorly, you’re essentially going to continue on that … unless you have recovery sleep for more nights.”

One possible biological mechanism underlying this lasting effect of sleep deprivation may be because of adenosine, a sleep-inducing substance that is left over from energy consumption and accumulates in our waking hours, Van Dongen said. Adenosine may account for the sleepiness we feel in short-term sleep loss and returns to baseline relatively quickly within a night’s rest.

But the chronic effects of sleep loss may be because of the accumulation of adenosine receptors. (Caffeine helps people stay awake because it temporarily blocks these receptors.) But sleep deprivation increases the number of adenosine receptors which in turn increases our sensitivity to sleep loss. It takes longer — days or about a week — for these adenosine receptors to return to baseline with adequate sleep, which may be why the effects of chronic sleep deprivation linger.

Sleeping in on the weekend is a “great idea,” Van Dongen said. “But it will not completely wipe away your sleep because you’re carrying your prior sleep history with you.”

Prioritize sleep

While Americans may be sleeping more than ever, many still do not get enough. We tend not to prioritize sleep as compared with all the other obligations and activities we need or want to do.

“I think it’s just that it’s so easy to put off,” Banks said. There are many ways to mask sleepiness — moving around, socializing, imbibing in caffeine.

“Naps are super helpful, and I recommend it to everybody who needs them,” Van Dongen said. “But don’t for a moment assume that that will take care of all you need because it won’t.”

Instead, we need to try to prioritize sleep when we can get it.

The only way to optimize our cognitive performance is “regular, consistent, sufficient sleep,” Lo said. “I know we struggle to do that. Just try to stick to that as much as you can.”