Right. Pay attention, because I'm only going to explain this once, and I do mean once, because the last time I explained sleep cycles to someone they fell asleep halfway through, which rather proved the point but was nonetheless insulting.
Your brain operates in 90-minute sleep cycles. Each one takes you from light sleep, down into the deep abyss where your body does all its tedious repair work, and then up into REM sleep, where you dream about being chased through a supermarket by your old maths teacher. This is apparently essential for memory consolidation. Don't ask me why the supermarket is necessary.
The PROBLEM — and do pay attention here — is that if some ghastly alarm clock wrenches you out of deep sleep mid-cycle, you will feel absolutely dreadful. You'll stumble about like a concussed wildebeest, wondering who you are and why you agreed to have a job. HOWEVER, if you time your waking to the end of a cycle, you pop up like toast. Same brain. Same bed. Entirely different human being. It's almost offensive how simple it is.
This calculator counts in 90-minute blocks and adds 15 minutes because that's roughly how long it takes you to stop checking your phone and actually fall asleep. Yes, we know it's longer. We're being polite. There's another 15 minutes added for waking up, during which you are legally neither asleep nor a functioning member of society. We don't judge. Much.
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