The thyrotropin (TSH) nycthemeral pattern is known to be strongly influenced by sleep, but previous studies have failed to demonstrate any link between sleep structure and TSH variations. Using 10-min blood sampling, nocturnal TSH profiles were analysed in 24 young healthy subjects during normal sleep. Six of the subjects then underwent a partial sleep deprivation experiment, sleep was permitted from 03.00 hours to 07.00 hours. Descending slopes of TSH values were observed for the first 20 minutes of SWS episodes, whereas no significant trend was found for other sleep stages. During the period of sleep deprivation, nocturnal TSH levels increased and then declined immediately after sleep onset; however, the association between SWS and descending TSH slopes persisted. This temporal concordance suggests that some particular mechanisms associated with SWS may modulate TSH release, or conversely that increasing TSH levels prevent the occurrence of SWS.