Sleep, Hormones, and Memory

Nocturnal sleep is characterized by a unique pattern of endocrine activity, which comprises reciprocal influences on the hypothalamo-pituitary-adrenal (HPA) and the somatotropic systems. During early sleep, when SWS prevails, HPA secretory activity is suppressed while growth hormone (GH) release reaches a maximum; this pattern is reversed during late sleep when REM sleep predominates. SWS benefits the consolidation of hippocampus-dependent declarative memories, whereas REM sleep improves amygdala-dependent emotional memories and procedural skill memories involving striato-cortical circuitry. Manipulation of plasma cortisol and GH concentration during sleep revealed an important role of HPA activity for memory consolidation. Pituitary-adrenal inhibition during SWS sleep represents a prerequisite for efficient consolidation of declarative memory; increased cortisol during late REM sleep appears to protect from an overshooting consolidation of emotional memories.