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The average sleep cycle is 1hr30mins. I think when we sleep for 6 hours, we complete 4 sleep cycles and wake up refreshed. When we sleep for 7 hours, we wake up mid-cycle. At ~8 hours, we wake up before we get stuck into the middle of a sleep cycle or enter REM.
I read this somewhere online, someone who knows more can verify if this is the actual science. If not, it's a fun hypothesis I'll choose to believe :p
Yes, that's true! And at least for me it is surprisingly accurate with 1h30. I started the habit of looking at the clock everytime I fall to sleep and wake up and it is always after 4.5, 6, 7.5 or 9 hours of sleep! If you want a simple app to calculate the ideal time to go to bed/time to wake up I can really recommend "Sleepy", available on F-Droid.
Also I heard that it is better to sleep 7.5 hours compared to 8 hours, because being waken up from deep sleep allegedly will make you more tired than these 30 minutes of extra sleep.