If you think you sleep enough by summing up hours from when you close your eyes to when you wake up, I'm here to tell you the truth. Very likely, not.
Well, keep reading and thank me later.
You've probably heard about sleep cycles. Every cycle lasts from 70 to 100 min at the beginning of sleep and from 90 to 120 min later. A healthy adult sleeps through 4 or 5 such cycles in the night.
Every cycle consists of 2 types of sleep: NREM and REM. And NREM consists of 3 stages. In a nutshell, we need NREM to recover during the night physically, REM - mentally.
As the body cycles through these stages of sleep, it dips from shallower to deeper states. Sometimes you don't even notice that you've been awake and right after, again, fall back into sleep.
So why would I say you may sleep less than you think?
Moving through the stages within each cycle and between the cycles, we all experience brief awakenings, so-called sleep disturbances (also known as sleep arousals).
Sleep disturbances are an essential component of the sleep process. Typically they last between 3 and 15 seconds, and usually, we don't remember them. Still, during the night, they accumulate, and we may lose up to 1 hour of sleep or more because of them.
It's normal for a healthy adult to be awake for 5% to 10% of the overall night's sleep. But frequently spending more than 10% of the time in bed awake could be a cause for concern.
To have an idea of what it may look like in reality, check the following data taken from my sleep tracker:
1st screenshot: fell asleep at 22:10, woke up at 07:02, time in bed 8:51, was awake 1:16h, overall slept 7:35
2nd screenshot: fell asleep at 22:30, woke up at 06:59, time in bed 8:28, was awake 0:39min, overall slept 7:49
Both cases show that even though I'm in bed for more than 8 hours, the actual sleep is less than 8 hours.
In the 2nd case, even though I was in bed later and woke up slightly earlier, my sleep quality was much better, so I gained more minutes of sleep.