Microsleep during partial sleep deprivation in depression. 
Sleep deprivation (SD) exerts a beneficial effect on mood and sleep in about 60% of depressed patients usually followed by a relapse into depression after the recovery night.  Short phases of sleepiness, especially naps in the early morning, may be responsible for this phenomenon.  To evaluate the effect of short, even ultrashort phases of sleep-microsleep (MS) during partial sleep deprivation (PSD) on mood, cognitive psychomotor performance (CPP), and sleep, an electroencephalograph (EEG) was continuously recorded over 60 hours in 12 patients with major depression.  Subjective mood was assessed by a visual analogue scale and CPP by a letter cancellation test.  The results illustrate that in depressed patients during PSD the amount of MS is increased, predominantly in the early morning, which was subjectively unrecognized and not observed by nursing staff.  Patients with a low cumulative amount of MS during PSD improved significantly in mood, CPP, and sleep pattern compared to the patients with a high amount of MS who showed only slight changes.  Therefore, accumulated MS may influence the SD-induced positive effects in depressed patients.