Spontaneous episodic thoughts about the past and future are common during waking but rarely occur during N2 or REM sleep. Analysis of thought reports from 138 participants who underwent experience-sampling while awake and serial awakenings during sleep shows that waking spontaneous thought frequently includes autobiographical planning with a strong bias toward the future. In contrast, dreaming sleep states rarely feature such mental time travel. This suggests that human consciousness differs substantially across the sleep-wake cycle in how it typically engages with episodic past and future events.
Slow-wave activity (SWA) during early non-rapid eye movement sleep is lower in people with treatment-resistant depression (TRD) than in healthy volunteers. Ketamine, but not placebo, increases SWA in TRD patients, especially those who respond to treatment, while having no effect on SWA in healthy volunteers. Ketamine also improves overall sleep in TRD patients by increasing total sleep time and sleep efficiency and reducing sleep latency. The increase in SWA after ketamine lessens with age. The findings suggest that ketamine's antidepressant effects are closely tied to its modulation of early sleep SWA and its ability to improve sleep architecture in TRD.