Anyone who has tried to pull a late-night study session and wound up rereading the same pages of their  textbook because  they can’t focus has experienced it.  And countless studies confirm it: if you’re sleep deprived, your brain starts functioning poorly. Reaction times slip, you’re more prone to careless actions, and 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 more or less sensitive to precisely how you 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, and you’ll necessarily be awake more—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.