Changes in sleep architecture following chronic mild stress.

Chronic exposure to mild unpredictable stress causes subsensitivity to rewards (anhedonia). These effects are reversible by chronic treatment with antidepressant drugs, and have been proposed as an animal model of depression. In the present study, sleep architecture, particularly the rapid eye movement (REM) component, was mapped in rats following exposure to chronic mild stress. The study used a unique large scale automated sleep system to record and analyze the sleep signals from 32 rats simultaneously. The effects of stress on sleep were maximal following 21 days of stress, at which time the stressed animals demonstrated decreases in active waking and deep sleep, and disruptions of REM sleep. The changes in REM sleep included increases in the duration of and transitions into REM sleep over the sleep part of the sleep-wake cycle, and most importantly, a reduced latency to the onset of the first REM period. These sleep abnormalities, and in particular the decrease in REM latency, are consistent with those reported in endogenous depression. The results provide further support for the validity of the chronic mild stress paradigm as an animal model to study the mechanisms underlying endogenous depression.