Yes, yawns are contagious, but that’s not the main reason we all seem sleepy.Yearning to yawn: the neural basis of contagious yawning. Schürmann M, Hesse MD, Stephan KE. NeuroImage, 2005, Apr.;24(4):1053-8119.”>Yearning to yawn: the neural basis of contagious yawning. Schürmann M, Hesse MD, Stephan KE. NeuroImage, 2005, Apr.;24(4):1053-8119. In a recent CDC survey, 35 percent of Americans said they get fewer than the recommended seven hours of shut-eye every night. And that’s a big deal considering we need sleep to rest, recharge, reduce stress, and even lose weight.

If you’re the type of person who’s clocking four to five hours of sleep (and probably pounding Red Bulls or cups of coffee) every weeknight, chances are you’ve tried to catch up on sleep by snoozing for a few extra hours on the weekend. But is it really possible to get back those lost zzzs?

Sleep Debt

Just like student loans, our bodies require regular payments—luckily seven to nine hours of sleep every night will do. If we start to sink below the seven-hour minimum, we fall into sleep debt. Over time, as that debt climbs, it becomes more and more difficult to catch up on sleep.

A few restless nights—what sleep researchers call acute sleep deprivation—is an easy debt to repay. Just snooze for three to four more hours than usual over the weekend, and you should be back on track.Effects of recovery sleep after one work week of mild sleep restriction on interleukin-6 and cortisol secretion and daytime sleepiness and performance. Pejovic S, Basta M, Vgontzas AN. American journal of physiology. Endocrinology and metabolism, 2013, Aug.;305(7):1522-1555.”>Effects of recovery sleep after one work week of mild sleep restriction on interleukin-6 and cortisol secretion and daytime sleepiness and performance. Pejovic S, Basta M, Vgontzas AN. American journal of physiology. Endocrinology and metabolism, 2013, Aug.;305(7):1522-1555. It’s much harder to catch up on sleep if you have chronic sleep deprivation—logging fewer than five hours for an extended period of time.

In one study, after sleeping for six hours per night for two weeks, study participants’ physical and cognitive abilities were impaired on a level similar to someone who had gone without sleep for two nights straight.The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Van Dongen HP, Maislin G, Mullington JM. Sleep, 2003, Jul.;26(2):0161-8105.”>The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Van Dongen HP, Maislin G, Mullington JM. Sleep, 2003, Jul.;26(2):0161-8105. But even when they were walking zombies, most people had no idea they were so sleep deprived. That foggy state becomes the scary norm.

The Takeaway

It’s possible to catch up on sleep if you’ve had a couple of rough nights. But the longer we go without zzzs, the harder it is to get ’em back.

Originally published December 2011. Updated August 2015.