Few things better illustrate how little we understand ourselves than the mysteries of sleep. For a start, there’s the big mystery of why we do it in the first place, since in evolutionary terms, being unconscious for a third of every day is one of the most suicidal strategies imaginable. (This means that whatever sleep’s for, it must be really important to make it worth the tradeoff of staying exposed to snakes and tigers all night long.) Then there’s the everyday mystery of how to make yourself do it when you can’t. (My top recommendation remains the visualisation method known as the “cognitive shuffle”: download the MySleepButton app, or just choose a letter of the alphabet and sequentially imagine objects beginning with that letter.) But our ignorance about sleep goes further: it’s also fairly likely you have no real clue how much sleep you got last night, or how sleep-deprived you are right now.

A six-hour night isn’t brilliant, I tell myself on the fairly regular occasions it happens, but surely it’s not too bad? After all, it’s only an hour less than the alleged ideal of seven hours, and 50% longer than the four hours to which Donald Trump reportedly confines himself. (Don’t think too hard about that, or it will disturb your sleep even more.) Yet in a study highlighted recently by Fast Company magazine, participants whose sleep was limited to six hours a night for 10 days performed just as badly on certain cognitive tasks as those who got zero sleep for two days straight. In other words, after a couple of weeks, six hours’ sleep is arguably as bad as none at all. Or even worse, in a way, because at least the zero-sleepers were able to rate themselves as extremely sleep-deprived; even by the time their alertness was just as poor, the six-hour sleepers still rated themselves less sleepy. Incremental sleep deprivation is like the water in the proverbial exercise of boiling a frog. By the end, it’s as bad as the sudden version, but the change is so gradual you hardly notice.

Making matters worse, you may well be getting six hours’ sleep, or some other insufficient amount, while believing you’re getting your full complement. That’s another standard finding of sleep studies: we’re constantly overestimating how much we get – by about 48 minutes on average, according to one study. Which is where things get complex, because researchers have also identified a “sleep placebo effect”: you do better on cognitive tasks when you believe you’ve slept well, even if you haven’t.

So the practical implications come down to this: above all, you probably need more sleep, even if you don’t feel sleepy, or you’re convinced you’re getting plenty. On the other hand, once you’ve taken that on board and adjusted your habits accordingly, please wipe the previous sentence from your mind, and do what you can to believe you’re well rested regardless. (For example, by not telling yourself how tired you feel.) You need more sleep and you need to believe you don’t. It’s a paradox, but don’t let that keep you awake.

oliver.burkeman@theguardian.com