When and how much should we sleep?
Most adults generally need 7-9 hours a day in bed (or the chair or wherever it is that they sleep), and will sleep 6-8 hours of this time. If they get less time in bed or less sleep, their performance is impaired and they may feel tired and sleepy during waking hours. If they get more time in bed, they will spend more time lying awake in bed.
Does it matter when they get their sleep? It depends. Some people can sleep whenever and adapt to any schedule easily. Other people have a harder time adapting to any new schedule. We seem to have evolved to sleep when it is dark and be awake when it is light. That means, especially in southern or northern latitudes, we may have evolved to sleep more in the winter and less in the summer. There is considerable evidence that we may also have spent much of the dark time resting, but may have slept in two phases, with a phase of wakefulness in between in the middle of the night (segmented sleep). There is also considerable evidence that we may have taken a nap in the middle of the day (and still do, in many cultures).
Our sleep habits probably changed when we settled down to agriculture from our hunter-gatherer days. They definitely changed again with the industrial revolution. When factories demanded constant work schedules, we had to adapt so we would get up early in the morning to get to the factory on time. We worked 12-16 hour days (yes, that was required in the beginning). Then we returned to our crowded homes to sleep several to a room in shared beds. The railroads (with time schedules for the trains) required standard time across different places, and made our sleep even more subservient to the clock. Compulsory school for children extended the regimentation of sleep to children.
Artificial light made it possible to keep factories and businesses humming 24 hours a day. It also made late night entertainment possible, whether it be reading, live entertainment, or later radio and television, and today the internet and computers. So we stayed up later, spent less time in bed, and began to sleep in one continuous phase instead of two without the middle of the night awakening. And there are those of us who end up with delayed sleep phase syndrome.
Many of us work late shifts or night shifts, which means we have to sleep when it is light and stay awake when it is dark. This in itself may not be a problem (after all, we can travel to the other side of the world and get used to the different time zone in a short period of time). However, we then travel across time zones every time we are off work (by sleeping when it is dark and staying awake when it is light), so we never quite get used to the shift work schedule. Some of us adapt easily, others end up with shift work disorder.
Now, when we get older and retire, and no longer have to answer to a work schedule, some of us spend too much time in bed. So we sleep 6-8 hours out of the 10-16 hours a day we may spend in bed, and lie awake the rest of the time. But we expect to sleep when we are in bed, so we complain of insomnia and end up addicted to sleeping pills, without realizing that is it normal to lie awake for hours in bed if we are going to spend much more than 7-9 hours a day in bed. Unless, of course, we have a disorder of excessive sleepiness which makes us sleep an excessive number of hours.