I think this topic is highly important to discuss.
It is said that Tesla used to sleep 15-20 minutes every 4 hours of a day, while Musk sleeps 6 hours a day as he says in his interview with Joe Rogan
How much do you sleep? How many hours is enough for you?
I slept 9 hours a night, every night, for most of my life. Was always a straight A student. Tons of energy. About 5 years ago, my age caught up with me. I gained some weight. Could not sleep more than 4 or 5 hours. Exhausted all the time with brain fog.
Lost most of the weight. Still toning and working on the last bit. My sleep has improved but I canāt seem to sleep past 4:30 am. I donāt really expect to return to my old self. I am not as young as I once was. Itās not all about sleep. Sleep is over rated. 
Apparently, up to (and beyond) the Tudor period, it was common for people to have two sleeps. They would generally go to bed earlier than we do, but would then wake up in the early hours. They might light a candle and do some reading, or engage in a little light housework. Apparently this was also considered the proper time for sex.
After an hour or two, they would doze off again, and then get up at dawn (or before, if you were a servant).
It seems this only stopped being normal behaviour with the introduction of reliable artificial lighting (gas, and then electric), which allowed people to stay up much longer in the evenings.
The idea of 8 hours sleep a night is not normal, and has only been relatively recently introduced.
OMG! I must be a Tudor at heart . 
I have done that, get up and clean house then go back to bed.
I generally sleep for 4 hour stints & have done so since I was an adolescent (so very long ago).
Sometimes a day may involve 2 of these stints yet other days, it may just be the 1 stint.
Sometimes I have an afternoon nap in preparation for an expected long night (sort of a fast charge).
If I was to guess, Iād say I get about 6 hours sleep every 24 hours on average but itās dependant on what I need to do & what I may be doing the following day, (Iām 95% retired now ), or indeed how much I did today.
Itās common for me to get up just on midnight & go do some tinkering on a project or some office work. Probably came from my need to study in my youth: Itās quiet & allows me to concentrate.
This often just leads to daylight & I just keep going.
Iām very much an early bird so I almost never sleep beyond dawn regardless of my sleep amount that night.
Maybe once every 3 months Iāll have a ābigā sleep where I do the solid 8 hours without waking, like a kind of deficit adjustment.
Interesting, I also heard people used to have the concept of first and second sleep. I guess we learn from that that sleeping in one block is not a general requirement.
I never do anything regularly and must have messed up my sleep cycle completely (for example by always having the radio or a stream on in the background, any loud advertisement, or neighbour slamming a car door wakes me up).
How do you guys even know when you fall asleep and how long? For example, now itās 5am ā I donāt recall when I woke up and when I started reading web pages. I couldnāt answer the question how many hours I sleep. If I āgo to bedā at, for example, 10pm and get up at 10am, that has nothing to do with āsleeping 12 hoursā. I canāt possibly check the clock all the time and then calculate the totalā¦?
I once had an app where you put the phone on the mattress and it tracked movements versus lying completely still (presumably deep sleep), and my results looked completely random. 
I have a Sleep Number bed. It tracks my restful, restless sleep and bed exits. But, I also have a clock nearby and see it everytime I wake up. My routine is fairly normal now. If I wake before 4 am, I read a bit, (it is 3:11 am now) and usually go back to sleep. If it is 4 or later, I read a bit then get up for the day by 6 am.
I have learned to embrace my shorter sleep time by getting a lot of reading done and using that time to study things I want to improve on or learn. Mentally, I am very alert early in the morning. By afternoon, I am practically useless.
Just habit over time, knowing when I chose to sleep & by a glance at the clock when I wake up. Pretty much can āfeelā how much time has passed anyway.
Same thing during the day: a glance around me at the ambient light places me within about 20 minutes of an accurate guess at the time.
Probably comes from a lifetime living on the land.
Same with directions. I just look around & Iām pretty accurate which way I need to go.
Interesting, that. I always know which direction home is. I donāt even have to look (I mean it works in the dark, or inside buildings). It was quite a surprise to me to discover that not everyone could do this. I must have been 16 or 17 when I found out it was unusual.
Itās not as useful as you might think. It only works with home - I canāt tell the direction to anywhere else, and it doesnāt take any account of distance or obstacles.
So I can be anywhere, and I can point and say āhome is that wayā, and it works. But it doesnāt tell me that thereās an ocean in the way, or that thereās 1,500 miles to walk, or that the route would take me over a cliff, or through a war zone. So unless Iām quite close to home to begin with, itās not particularly helpful.
Iām ok if Iām outside. Doesnāt work for me inside thoughā¦
Put me in a large shopping mall or hospital labyrinth & Iāll be lost in a few moments.
Unless I āmapā my position by shops / distinctive objects Iāll wander around confused until I stumble across an exitā¦& make my escape. 
I love maps, which is a good thing. Without one, I am lost. Give me a map and point me N. Then I can get anywhere.
Iāve had a sleep disorder my entire life. It is the most unpredictable thing ever. I can expect to go to sleep for 8 hours and wake up 1 hour later or 12 hours later. I just took a nap at 5pm expecting to only sleep for an hour or two - I woke up at 1am. Sometimes I canāt sleep at all for days.
Mostly Iām nocturnal - from what Iāve read thereās evolutionary evidence of both habitual nocturnal and diurnal homo sapiens. Itās unnatural to expect people to follow the patterns of modern society - going to school, working 9 to 5, etc.
That makes a lot of sense if youāve ever lived with nothing but a wood stove for heating. In winter, the rythm of waking up early in the freezing cold, getting dressed like an onion, lighting the fire, do a bit of other stuff until the fire really gets going, and then crawling back into the cozy warm bed and sleep another stretch until the room has warmed up develops pretty naturally.
Currently though, I go on 5.5 to 6 hours of sleep during the night, and usually 1 or 2 10-minute powernaps during the day. Iāve learned in the military to fall asleep anywhere, in any position at any noise level at any time almost immediately if Iām not wide awake, so that works out pretty well.
Yeah, uhmā¦ If I look around, I have to be really careful to even remember the original way I was facing afterwardsā¦ 
Here a very recent article about the above topic of biphasic sleep. They say in the end that there is no reason to assume that one is right/better and the other wrong/worse. Mostly the news is for people with natural biphasic sleep, that itās not a disorder.
In scifi (to get back to the topic of this forum) 
I am surprised how Little the matter of time synchronisation between spaceships and planets etc is mentioned. What are their natural rhythms? Does any scifi story ever say, āwell obviously you didnāt meet any X ā at this time period!ā 