Sleep, science tells us, is a lot like a bank account with a minimum balance penalty. You can short the account a few days a month as long as you replenish it with fresh funds before the penalty kicks in. This understanding, known colloquially as “paying off your sleep debt,” has held sway over sleep research for the last few decades, and has served as a comfortable context for popular media to discuss sleep with weary eyed readers and listeners.
The question is – just how scientifically valid is the sleep debt theory?
Recent research targeted this question by testing the theory across a few things that sleep, or the lack of it, is known to influence: attention, stress, daytime sleepiness, and low-grade inflammation. The first three are widely known for their linkage to sleep, while the last—inflammation—isn’t, but should be. Low-grade tissue inflammation has been increasingly linked to a range of unhealthiness, with heart disease high on the list.
Study participants were first evaluated in a sleep lab for four nights of eight-hour sleep to establish a baseline. This provided the researchers with a measurement of normal attention, stress, sleepiness and inflammation levels to measure against.
The participants then endured six nights of six-hour sleep (a decent average for someone working a demanding job and managing an active family and social life). They were then allowed three nights of 10-hour catch-up sleep. Throughout the study, participants’ health and ability to perform a series of tasks were evaluated.
Sleep debt theory predicts that the negative effects from the first six nights of minimal sleep would be largely reversed by the last three nights of catch-up sleep – but that’s not exactly what happened.
The analysis showed that the six nights of sleep deprivation had a negative effect on attention, daytime sleepiness, and inflammation as measured by blood levels of interleukin-6 (IL-6), a biomarker for tissue inflammation throughout the body — all as predicted. It did not, however, have an effect on levels of the stress hormone cortisol—the biomarker used to measure stress in the study—which remained essentially the same as baseline levels.
After three-nights of catch-up sleep, daytime sleepiness returned to baseline levels – score one for sleep debt theory. Levels of IL-6 also returned to baseline after catch-up – another score in the theory’s corner. Cortisol levels remained unchanged, but that’s not necessarily a plus for the theory (more on that in a moment).
Attention levels, which dropped significantly during the sleep-deprivation period, didn't return to baseline after the catch-up period. That’s an especially big strike against the theory since attention, perhaps more than any other measurement, directly affects performance. Along with many other draws on attention—like using a smart phone while trying to drive—minimal sleep isn’t just a hindrance, it’s dangerous, and this study tells us that sleeping heavy on the weekends won’t renew it.
Coming back to the stress hormone cortisol, the researchers point out that its level remaining relatively unchanged probably indicates that the participants were already sleep deprived before they started the study. Previous research has shown a strong connection between cortisol and sleep; the less sleep we get, the higher the level of the stress hormone circulating in our bodies, and that carries its own set of health dangers. This study doesn’t contradict that evidence, but also doesn’t tell us one way or the other if catch-up sleep decreases cortisol levels.
The takeaway from the study is that catch-up sleep helps us pay off some, but by no means all of our sleep debt. And given the results on impaired attention, another takeaway is that it’s best to keep your sleep-deprived nights to a minimum. Just because you slept in Saturday and Sunday doesn’t mean you’ll be sharp Monday morning.
You can find David DiSalvo on Twitter @neuronarrative and at his website, The Daily Brain. His latest book is Brain Changer: How Harnessing Your Brain’s Power To Adapt Can Change Your Life.