Twelve older (aged 60-85) and twelve younger (aged 18-35) healthy male and female volunteers will participate in the study. The investigational procedures will include medical screening (e.g., physical, blood and urine tests, and bone mineral density assessment), home monitoring of sleep schedules for three weeks, and three overnight visits in the laboratory spaced approximately one week apart. During the laboratory visits, participants will be awakened in the middle of the night to perform walking stability and cognitive function tests. Before going to bed, participants will take either a placebo pill or a sleeping medication. On one of the visits, participants will be asked to go to bed about 2 hours later than usual.