The Circadian cycle controls when we sleep and when we wake up, but there is much more to it than that. Melatonin is produced at points in the cycle, and bowel movements are encouraged or suppressed depending on where a person may be in their rhythm. One's personal cycle has great tendency to vary, but nearly everyone feels more comfortable, healthy and happy when they perceive their own personal rhythms and nurture them to the best of their ability.
The best way to do this is to make sure that you get enough sleep. There is no chemical that you can take that will accomplish this for you. Instead, it is important to abstain from caffeine and other stimulants before it is time to sleep. Sleeping at roughly the same time and for the same number of hours every night can be broadly useful. The sun is a powerful force in orienting one's circadian rhythms, and if it is possible to spend a lot of time outdoors and in the sun, then it will make it all the easier to get up at roughly the same time and go to sleep at roughly the same time. However, this is not always possible due to seasonal restrictions, and it is not always necessary. Artificial lighting can be used to change the cycle, although it is far more common for this lighting to accidentally interfere with the existing cycle. For example, sleeping in a room with bright lights has been shown to disrupt circadian rhythms.
The most important thing is to observe your body's rhythms and work with them instead of against them. Even a month or two of taking your sleep schedule seriously, protecting it and working with it can make an enormous difference in repairing your circadian rhythms and improving your quality of life.