I expected a strong biological preference for amount of sleep and for the sleep cycle (how often we need sleep) but didn't expect there to be a strong biological cause for various sleep timings.
I was wrong: there does seem to be a biological cause for sleep timing (and it can change with age). However, most people aren't strong early birds are or night owls (estimates of people with strong preferences range widely, from 15% up to 50%, although most are around 30%), even though people commonly overestimate their preference. For those that aren't, sleep patterns can vary up to 2 hours on either side and people can shift sleep cycles (presumably without ill effect, although I haven't found good information on exact effects). Also, there are many external factors, social and biological that can shift sleep cycles.
Based on my limited research, my findings are:
- There definitely is a biological preference for being early bird or night owl.
- For a minority of people, this preference is very strong and cannot be shifted without problems
- For most people, sleep timing can be shifted within reason (within 2 hours) without problems (but I don't know if minor shifts affect alertness, energy, etc and if so, by how much).
- For most people, social and biological factor (job, friends, school, diet, exercise) do shift sleep timing slightly.
- Being on a consistent sleep cycle (not disrupting the circadian rhythm) and getting enough hours of sleep are more important to sleep than the exact timing (again, within a few hours).
Let's say that without an alarm, I will currently sleep in to 10AM. If I wake up at 8AM for a few days, I'll feel pretty bad. But is that because I'm genetically a night owl or because I've disrupted my circadian rhythm and haven't gotten enough sleep? It's very hard to say. The only way to tell for sure would be to get up at 8AM for a month and see if there were ill effects. It may be true that 8AM or 9AM or 11AM are my optimal wake times, but this has been masked by my habits.
There is undoubtedly a biological factor in sleep timing and it varies on age and depending on the person. If a person currently wakes at noon, shifting to wake at 6AM is going to cause problems. However, if a person currently wakes at 10AM, it's he or she may have no problems getting up at 7 or 8AM (after an initial unpleasant transition time) and it may actually be more optimal.
Anyway, it's pretty interesting stuff (hence my way too long response :)). If anyone finds more information, tweet me.
http://www.wisegeek.com/what-is-a-chronotype.htm
http://en.wikipedia.org/wiki/Chronotype
http://healthysleep.med.harvard.edu/healthy/science/variations/changes-in-sleep-with-age
I would agree with these findings based on my own experiences and observations. I'll add my own perspective though it's mostly experiential.
I am definitely a morning person. How early I rise depends on when I go to bed. I usually get to bed between 11pm and 12am. I naturally wake between 6 and 7am, no alarm. Interestingly, if I go to bed too early, like 10pm, I have trouble getting to sleep right away. That is unless I had an exhausting day. If I go to bed really late, like 2am, I have difficultly sleeping past 8am.
My wife goes to bed at the same time but would happily stay up later. She struggles to get up before 7am. She would be the type to hit the snooze button multiple times and almost can't just get up. She will be groggy and is the type that 'needs' her coffee to be really awake. I rise instantly and can be fully functional shortly afterwards. I also don't do caffeine. It seems both of us have this waking pattern all our lives.
I would say that I am biologically geared to be this way but in different phases of my life I could easily change this pattern. As a teenager I often would stay up until 4am or dawn and sleep until 1pm or so. I could get up early but really didn't like it at all and never naturally did it. In the Army (19-21), I got used to getting up at 6am and had to go to bed early so I could be well rested. I was incredibly active during those years and I got used to that pattern which I think helped influence my current schedule. In my early-mid 20s, I was a musician and often times was out until 3am many nights a week. I slept until 10ish during those days. The other significant shift was when we had children. Children do not sleep in and as a parent you simply cannot sleep in anymore. By 6am, someone was awake in the house. Naps became so precious as well as an early bedtime. After having two children, I cannot comfortably sleep in past 8am. I simply don't enjoy that sleep and just suffer the effects of not getting enough sleep for that day.
Another observation is for most of my adult life I would get between 5-6 hours of sleep a night. It may have fine when I was younger but now I can't do that for very long. In the Army they purposefully sleep deprived us. They only had to give you 4 hours of sleep a night and it did not have to be contiguous. This really only mattered when we were in the field. Back in the barracks they don't care when you go to bed but there is a 6am first call with a 6:30 formation. Once I hit my 30s I found I couldn't stay up as late and would start to 'crash'. Sure I could stay up if I were active but if I slowed down, I'd start nodding off. The other thing to note about the 30s was I really did best when I got around 7 hours of sleep. I couldn't really sleep more and going under 6 would have noticeable effects.
One final note. For a short while I adapted my lifestyle to be more Ayurvedic. It's a fascinating subject that touches almost all parts of human life. It's ancient Indian in origin and yoga comes from it. They had some beliefs on sleep and they said the optimal human sleep time was from 9pm to 6am. If you followed that schedule then your body would be at its healthiest from a sleep standpoint. Though I struggle to go to bed that early, I do agree with their assessment that rising closer to 6am (assuming you got enough sleep) makes you feel your best.
Here are a couple links on Ayurvedic sleep:
http://www.ayurbalance.com/explore_articlerestfulsleep.htm
http://www.mapi.com/ayurveda_health_care/self_care/sleep.html