Circadian rhythms are 24-hour cycles that influence natural functions in the body such as heart rate, blood pressure, and body temperature. Circadian rhythms provide the body with an internal clock and affect sleep patterns. The purpose of this study is to determine the effects of vitamin B12 supplements on circadian rhythms and sleep-wake regulation.

Individuals with circadian rhythm sleep disorder suffer from recurrent patterns of disrupted sleep that can significantly affect their daily functioning. Evidence suggests that vitamin B12 supplements may have a beneficial effect on sleep patterns. However, research on the effects of vitamin B12 supplements on sleep is limited. This study will determine the effects of vitamin B12 supplements on circadian rhythms and sleep-wake regulation.

This study will last 47 days. Participants will undergo a 3-week medical and psychological screening prior to study entry. Eligible participants will be admitted to the General Clinical Research Center where they will live in a private study room for the duration of the study. The room will be a time-free environment with no windows, clocks, television, or radio. Participants will not be able to make or receive telephone calls, and their bedtimes, wake times, and mealtimes will be determined by study researchers. Participants will be randomly assigned to receive three capsules of either vitamin B12 or placebo daily. Blood and saliva collection will occur daily to determine core body temperature and hormone levels. Performance tests to determine participants' cognitive abilities will be administered at regular intervals throughout each day. Participants' sleep will be recorded every night with polysomnography, which will monitor brain and muscle activity and breathing patterns during sleep.

Inclusion Criteria:

• Diagnosis of delayed sleep phase syndrome OR self-described as a "night owl"

Exclusion Criteria:

• History of medical illness
• History of psychiatric illness in participant or his or her family members
• Current medication or vitamin use
• Follow a vegan diet
• Significant visual problem
• Recent travel across time zones
• History of rotating shiftwork