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Why is it impossible to go to sleep, only when one must quickly get as much sleep as possible for the next day -- like the day before hiking, or after staying up through the night for an exam?

I must sleep. I must sleep. This forced idea would only force one awake, that's for sure. However, if one tries to hint oneself 'Don't think I must sleep,' what waits ahead is just the unescapable bottomless swamp of sleeplessness. One will just sink deeper and deeper, and find no way to escape.

This effect manifests itself in the opposite way during times that one must not fall asleep, like in a lesson. I must not sleep. I must not sleep. Such commands will somehow be interpretted in the brain as urge to sleep.

So why not try giving the opposite command this time -- "I must not sleep" when early sleep is needed?

Unfortunately, the idea "I must sleep to prepare for tomorrow" was already stuck in a corner of the brain, so as expected, there is not way such a command could help with sleeping.

Because of that, when Yumi set foot in her dreams that night, it was already around two hours after she got into bed. Three hours was left before the alarm clock would be ringing.