Nineteen-hour variation of postural sway, alertness and rectal temperature during sleep deprivation were studied. Alertness decreased gradually at night and remained low until morning. Postural sway in the eyes-closed condition increased during early morning. In six of the eight subjects the greatest sway was observed during the 3-h period when rectal temperature was at its minimum. It is suggested that unbearable sleepiness during sleep deprivation will give rise to measurable impairment of postural balance especially during the time zone of temperature nadir.