Bedtimes and wake-up times vary from person to person. The aim of this research is to examine the associations between sleep habits and metabolism in healthy people. In this study, the investigators will collect information on sleep-wake habits and how the body uses energy.

Inclusion Criteria:

• Age: 21-40 years
• Body mass index: 25.0 to 29.9 kg/m2
• Average habitual sleep duration of 6.5 hours or less per night (as confirmed by actigraphy)
• stable sleep habits for the past 6 months

Exclusion Criteria:

• obstructive sleep apnea or history of any other sleep disorder
• night or rotating shift work (current or in the past 2 years)
• habitual daytime naps
• recent (< 4 week) travel across time zones
• extreme chronotypes
• any acute or chronic medical condition
• diabetes
• prior or current eating or psychiatric disorders
• claustrophobia
• irregular menstrual periods, menopause, pregnancy,
• alcohol abuse, excessive caffeine intake, smoking, illegal drug use
• subjects who are currently following a weight loss regimen or any other special diet or exercise programs
• subjects who have received iv or oral contrast in the past 2 weeks
• use of any prescription medication that can affect sleep or metabolism with the exception of antihypertensive and lipid lowering agents as follows. Subjects who are taking anti-hypertensive medication and/or lipid-lowering agents will be included only if they are well controlled and on a stable regimen (no change in medications in the previous 3 months). However, beta-blockers and thiazide diuretics will not be allowed, as these medications are known to affect insulin sensitivity. Women will be required to not be on hormone replacement therapy.