wants to give you a better night’s sleep through REM cycles. Scrape away its JavaScript veneer, and you’re left with a calculator — specifically, one that can only count back in 90-minute intervals.

If you tell you have to get up at 7 a.m., for instance, this is what it spits out: is based on the concept that every hour and a half, humans enter rapid-eye-movement sleep. REM cycles come with a whole host of physiological change: You experience muscle atonia or, colloquially, sleep paralysis; your eyes flicker; your brain’s learning centers are active; and you dream. An alarm dragging you awake out of the throes of REM sleep sucks. wants you to avoid this by going to sleep around the times it calculates (and get in bed 14 minutes beforehand, it says, in order to get snoozing on time).

Except the every 90-minute average is just that — an average. They may change from person to person, and even among individuals, REM cycles lengthen as the night goes on. And T-minus 14 minutes to sleep after you hit the hay — well, that’s an average, too, and mileage may vary.

Without a sleep lab on hand, it’s almost impossible to get a sense of what your cycles look like. By way of a horribly designed experiment with an n of 1, a week of alternating between an 11:30 p.m. sleep time — which is what recommended — and the not recommended 12:30 a.m. resulted in a self-reported groggier waking period. What can we take away? Not much, except that 7.5 hours of sleep is more refreshing than 6.5.


But isn’t a total failure. Sleep hygiene comes with cultivating good habits — and if encourages you to keep a regular pattern, or at least gets you to be more cognizant of when you should hop into bed, then calculate away.

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