Sleeplessness can hurt our memory, increase our stress levels, and even seriously impair our judgment. We all know it’s bad to go for long periods of time without getting enough sleep, but is it because of the long periods of wakefulness or the short periods of sleep? While the question may sound silly, it’s stumped scientists. But a new study sheds some light on the question.

In a paper published Monday in the journal Proceedings of the National Academy of Sciences, a team of researchers devised a way to tease apart the wakefulness-to-sleep ratio of chronic sleeplessness. In a small study of 17 participants, researchers had healthy volunteers live for 32 days in an unusual pattern of 20-hour days, which each included 15 hours of wakefulness and five hours of sleep. This allowed the researchers to test the effects of short sleep periods without the complicating factor of long wakefulness periods. This way, they could test whether short sleep cycles affected people’s cognition and behavior independently from long days spent awake.

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Chronic sleep restriction can decrease cognitive performance, even without long periods of wakefulness.

Compared to a control group that slept eight hours after being awake for 16, the researchers found that the 20-hour-day test subjects showed slower reaction times, lowered vigilance, and higher self-reported sleepiness when they had only slept five hours, even though they’d only been awake for 15 hours. This suggests that it is actually the sleep deficits, rather than being awake for a long time, that make us feel groggy and perform poorly when we go long periods without enough sleep. It’s a fairly simple answer to a long-vexing problem.

Additionally, because the 20-hour days don’t sync up with calendar days, the experimental periods spanned multiple days, transcending the normal circadian rhythm that our bodies are attuned to. The researchers found that the cognitive deficits associated with chronic sleep restriction were particularly pronounced during the normal circadian night, suggesting that even when human bodies are forced into weird 20-hour days, they still function as if they’re on a 24-hour clock.

“These data demonstrate that chronic short sleep duration, in the absence of extended wakefulness, impairs neurobehavioral performance, and that these impairments are worse during the circadian night,” write the study’s authors. This study represents a step toward better understanding the long-term effects of sleep disorders in humans, conditions that affect an estimated 50 to 70 million Americans.

So whether you work strange hours or you just play a lot of Fortnite late at night, sleeping for too little time seems to be bad for you. So do yourself a favor, give yourself a break, and get your eight hours of sleep.