Anyone who has tried to pull a late-night study session and wound up rereading the same pages of their textbook because they couldn't focus has experienced it. And countless studies confirm it: if you're sleep deprived, your brain starts functioning poorly. Your reaction times slip, you are more prone to careless actions, and you generally just get bad at things. But how is it your body registers "too little sleep"? It could be after you spend too much time awake. Or it could be the amount of sleep you get in a 24-hour period. Or it could be tracked in relationship to your body's internal 24-hour circadian clock.

A new study out this week suggests it's not just one of these things, and different aspects of our mental capacities are either more sensitive or less sensitive depending on how we end up short on sleep.

Deprived

The challenge with separating out different aspects of sleep deprivation in the real world is that anything you do will involve multiple aspects of sleep. Get too little sleep during a 24-hour cycle, which means more awake time—and awake at times your circadian clock says you shouldn't be. So, the researchers behind the new work messed with people's clocks. They got a small group of people (because it would be hard to recruit a large one) to live at a sleep center for 32 days, cut off from any indication of outside time.

Once inside, the participants were split into two groups, with eight people in the control group and nine in the experimental. Then, with a healthy dose of naps, everyone was shifted to a 20-hour sleep and wake cycle. The control group slept for nearly seven hours a day, or one-third of the day, the equivalent of eight hours of sleep in a 24-hour day. The experimental group was sleep deprived, getting less than a quarter of their day to sleep. That's 4.7 hours sleeping, in this case, which means they didn't spend more than 15 hours awake at a time—less than most people on a 24-hour cycle would.

The key thing about this is that nobody, not even the sleep-deprived individuals, ever spent that much time awake in a single stretch. And none of them ended up experiencing the 24-hour day that their circadian clocks expected.

Throughout this, the participants were given cognitive tests that tracked their reaction times and ability to focus attention. They were also asked to self-report how alert or sleepy they felt (that was two separate questions, but they did anti-correlate).

Feeling sleepy?

Even though none of the individuals in the experimental group was experiencing long periods of time awake, they still ended up sleep deprived. Over the course of the study, both their reaction times and ability to focus attention gradually lowered. That tells us it's not the length of time awake our bodies care about. Oddly, neither group reported that they felt unusually tired, even though one was clearly suffering a bit.

But things got a bit complex when the analysis included consideration of the 24-hour circadian clock. When it came to attention, this didn't matter—the experimental group performed equally poorly when their 20-hour clock left them taking the test during their circadian day or their circadian night. But when it came to reaction time, those subjected to sleep deprivation performed nearly the same as controls when tested during the circadian day. All their bad performances came when their internal clocks would be telling them it was the middle of the night.

The study is clearly limited by its very small size, and there is the issue that those subjected to sleep deprivation tended to fall asleep much faster, and thus spent less time lying awake. But, if anything, this should have limited the differences seen between the two groups, suggesting that the results could be even stronger when actual time spent asleep is considered.

On the more positive side of things, this isn't the first time this group has run long-term sleep studies like this, and their previous results also indicate sleep deprivation is complicated. A different set of participants was stretched out to a 43-hour day, and the sleep deprivation condition only allowed them 10 hours of sleep, which was followed by an epic 33 hours awake. Here, they were able to track how participants' performance on attention and reaction time tasks declined by the hour as they spent more time awake.

Yet a single 10-hour dose of sleep was sufficient for them to fully recover to normal performance before starting the decline all over again. Studies performed elsewhere indicate it's the total amount of sleep in a 24-hour period that matters—even if seven hours of sleep is broken up into two short chunks, it's enough to keep people's performance at normal levels.

All of this is still a bit confusing, in that we haven't completely isolated the effects of all the factors at play—total sleep, consecutive wakefulness, mismatched circadian clocks, and so on. But the results hint that different consequences of sleep deprivation are sensitive to different aspects of our sleep cycle. It really looks like there's no one thing that we can call sleep deprivation.

PNAS, 2017. DOI: /10.1073/pnas.1706694115 (About DOIs).