To examine the physiological role of melatonin in sleep, nocturnal melatonin secretion was suppressed using 100 mg oral atenolol in two studies. In Study 1, nocturanl sleep was recorded in 8 young men over 4 nights. Subjects received atenolol on one of the last 2 nights and showed significanly increased total wak time (TWT) and wakefulness after sleep onset (WASO), as well as decreased REM sleep and slow -wave sleep(SWS). When melatonin (total 5 mg) was given after atenolol on the other night, the changes in TWT, WASO, REM, and SWS were reversed. Sin Study 2, sleep onset latencies (SOL) and core temperature (Tc) of 10 young men were measured for 3 nonconsecutive nights. In a cross-over design, atenolol given on one night significantly suppressed urinary 6-sulphatoxymelatonin (6s-aMT) production and increased hourly measures of Tc and SOL relative to baseline night values. Oral melatonin (3 mg), administered after atenolol, reversed the changes in Tc and SOL. These results suggest that endogenous melatonin may assist in the maintenance of normal sleep architecture (Study 1) and also increase nocturnal sleep propensity by hypothermic effects (Study 2).