Studies on circadian rhythms suggest that sleeping 10 PM to 6 AM will give the most returns. This considers not only when the body appears to be the most geared for sleep, but also when the body is ready for other activities in the day.
For example, it appears we are most alert around 10 AM. It has been suggested that this time is ideal for performing your most challenging tasks of the day. The late afternoon appears ideal for physical exercise. Being awake and rested, and on a schedule, can help you take advantage of your highs and lows throughout the day.
Studies of the health impacts of working night shift have shown various negative factors, including higher risks of developing chronic health problems. A well known 2001 study suggested that women who worked graveyard shift had a significantly higher risk for developing breast cancer, due to exposure to light, suppressing the normal nocturnal release of melatonin, which in turn has an impact on estrogen release.
A BBC Horizon episode "The Secret Life of your Body Clock" attempts to answer the question of what we should be doing at what times of the day, in regards to our internal "body clock."
(The complete episode may be found on Youtube. Search: "BBC Horizon body clock")
Morning larks and night owls
The majority of people appear to be "morning larks," who optimally would sleep at 10 PM and rise at 6 AM. However, it is estimated that 10-20% of the population are what are referred to as "night owls," people who are more mentally alert later in the day. Night owls are reported to be "less happy" than morning larks, often waking with a scowl in early hours.
By age 60, most people are morning types, the researchers found. Only about seven percent of young adults are morning larks, but as the population ages, this switches -- in the older years only about seven percent of the population are still night owls.
Since light exposure appears to be the trigger for many of our biological processes, this article suggests a regular waking time and exposure to natural light in the early morning could help "convert" a night owl into a morning lark. The reason for the difference is not known.
Further reading on delayed sleep phase syndrome: