Sleep, hormones and memory

Abstract

Nocturnal sleep in humans is characterised by a unique pattern of endocrine activity. Secretory activity of the hypothalamo-pituitary-adrenal (HPA) system is maximally suppressed during the early part of sleep, which is dominated by slow wave sleep (SWS). Concurrently, release of growth hormone (GH) and prolactin reach a maximum. During the late part of sleep when rapid eye movement (REM) sleep is predominant, HPA secretory activity distinctly increases while the release of GH reaches a minimum and also prolactin concentrations decrease. Release of thyroid stimulating hormone (TSH) usually peaks in the evening and is suppressed during sleep. The regulatory effects of sleep and circadian rhythm on HPA secretory activity are synergistic so that sleep amplifies the effects of circadian oscillators. In contrasts, the effects of sleep and circadian rhythm on the release of TSH are antagonistic. The effects of sleep on the release of GH and prolactin appear to be rather independent from circadian rhythm. What are the functions of this sleep specific regulation of endocrine activity? Considering the essential role sleep plays for the consolidation of memories, in an ongoing series of studies we are testing whether the endocrine regulation during sleep serves to enhance the memory function of sleep. The first studies focussed on the role of pituitary-adrenal activity. SWS-rich early sleep has been demonstrated to benefit particularly the hippocampus dependent declarative memory system (storing facts and episodes). In contrast, REM sleep-rich late sleep appears to improve in particular procedural memory for skills, not depending on the hippocampus. Enhancing plasma glucocorticoid concentrations during SWS-rich early sleep completely blocked hippocampus dependent declarative memory consolidation, but did not affect hippocampus independent procedural memory. Also, procedural memory consolidation was not impaired when the rise in plasma cortisol concentration during late REM-sleep rich sleep was prevented by administration of metyrapone. The data underscore the importance of pituitary-adrenal inhibition during early SWS-rich sleep for efficient consolidation of declarative memories.