Converging evidence suggests that the regulation of emotional processes is an important function of sleep and dreaming. Early content analyses of REM sleep dreams showed that many dreams are highly emotional, with unpleasant emotions prevailing (Hall & Van de Castle 1966; Snyder 1970). This is in line with neuroimaging studies of REM sleep, demonstrating that neural areas involved in emotion regulation like the amygdala, medial prefrontal cortex, and anterior cingulate cortex are highly activated during REM sleep (Nir & Tononi 2010). Several REM-sleep characteristics differ between healthy subjects scoring low in depression scales and those with higher but still sub-clinical depression scores (Cartwright et al. 1998). After highly emotional life events, REM sleep changes can be observed in those subjects that react with symptoms of depression (Cartwright 1983), and dreams of depressed subjects differ from patients in remission (Cartwright et al. 2006). Likewise, in depressed patients the distribution of rapid eye movements in REM sleep differs in nights after which mood is estimated better than in the preceding evening compared to nights after which mood is unchanged (Indursky & Rotenberg 1998). It was therefore proposed that REM sleep dreaming serves as a mood regulation system and that a disturbance of this process might play a role in the development of affective disorders (Cartwright 2011). Changes in REM sleep are symptomatic of affective disorders and the sleep-memory relationship is altered in these diseases (Dresler et al. 2014). In healthy subjects, the consolidation of emotional texts (Wagner et al. 2001) or pictures (Hu et al. 2006; Nishida et al. 2009) is enhanced through REM sleep, an effect that has been shown to last for several years (Wagner et al. 2006).
While at first sight it might look as if REM sleep unequivocally strengthens emotional memory processes, some studies suggest a more complex picture: referring to the fact that emotional experiences are remembered better than neutral ones, however their emotional tone during retrieval decreases with time, it was proposed that REM sleep serves an emotional decoupling function: we sleep to remember emotionally-tagged information yet at the same time to forget the associated emotional tone (Walker & van der Helm 2009). While some studies support this model (Hu et al. 2006; Nishida et al. 2009), others suggest that the affective tone of emotional memories is preserved rather than reduced during REM sleep (Groch et al. 2013).
Besides negative emotions, sleep and dreaming have also been associated with positive affects. Recent dream report analyses suggest that positive emotions in dreams have been underestimated in previous studies and might be even more common than negative emotions (Malcolm-Smith et al. 2012; Sikka et al. 2014). In addition, the processing of reward has been associated with REM sleep and dreaming. For example, the expectancy of a reward enhances memory consolidation processes during sleep (Fischer & Born 2009), and reactivations of neural activity related to a reward-searching task have been observed in reward-related brain regions such as the ventral striatum during sleep (Pennartz et al. 2004). Instead of a simulation of purely aversive content such as threats, according to this account sleep favors the activation of representations of high emotional and motivational relevance in general (Perogamvros & Schwartz 2012, 2014).
In summary, a second important function of sleep and dreaming is the regulation of emotions, including both an enhancement of emotionally-tagged information and a decoupling of this information from its associated emotional tone.