Dreaming is hard to study in non-human animals because it is usually identified through verbal reports. Parallels between human dreaming—its phenomenology, physiology, and behaviors—offer a way forward. Three alternative measures of human dreaming (neural correlates, memory replay, and dream-enacting behaviors) can be applied to animals. Mammals and birds, with brains similar to humans, are good models for neural and memory measures. Cephalopods, especially octopuses, may be especially suitable for studying dream-enacting behaviors.
Dreams from the early night are more clearly connected to waking-life activities, while late-night dreams are more emotional, important, varied in time orientation, and hyperassociative. These differences align with known sleep-stage functions: non-REM sleep supports episodic memory consolidation and REM sleep supports emotional regulation and creativity. The findings suggest that dream content reflects underlying sleep processes such as memory consolidation, emotion-processing, and creativity across the night.
Incorporation of waking-life events into REM sleep dreams is highest on the night after and then 5-7 nights after the events, known as the day-residue and dream-lag effects. In experiment 1 with 44 participants, the dream-lag effect appeared for personally significant events incorporated into REM dreams recalled at home, but not for major daily activities or major concerns. No dream-lag effect was found for slow wave sleep dreams or for REM dreams collected in the lab after earlier SWS awakenings. In experiment 2, the dream-lag effect appeared for references to the sleep lab experience in home dreams only among participants who had prior concerns about being in the lab.