Why a calm, uninterrupted REM night may settle the amygdala
The amygdala is a small, almond-shaped cluster of brain regions involved in tagging things as emotionally significant. Part of what a good night's sleep seems to do is let the emotional charge of the previous day fade, so the same trigger packs less of a punch the next morning. One proposal for how this works comes from animal work: during REM sleep, a brainstem area called the locus coeruleus, a major source of the alerting chemical noradrenaline, is supposed to go quiet. If that quieting is incomplete, the theory goes, the brain's overnight reorganization of emotional circuits is impaired [1].
In people, restless REM sleep is thought to be a sign that this quieting is not happening properly, and it shows up across several psychiatric conditions [1]. The key distinction is between how much REM you get and how broken up it is. In the human imaging study, amygdala reactivity to a self-conscious emotional trigger fell overnight in proportion to the total duration of consolidated, uninterrupted REM sleep. Restless REM sleep, by contrast, blocked that overnight adaptation [1]. So the useful contrast is consolidated versus fragmented REM, not simply more versus less REM.
What a broken REM night looks like the next day
The next-day consequence is not just a vague bad mood. When REM sleep was selectively suppressed in a controlled study, participants showed increased negative affect and stronger amygdala responses to social exclusion the following day [3]. That fits the adaptation picture: if the overnight settling of emotional reactivity is prevented, the amygdala reacts more strongly to a socially painful event.
The 2019 study went a step further by experimentally nudging the process. Using targeted memory reactivation, in which an odor previously paired with the emotional stimulus was reintroduced during sleep, the researchers could strengthen both the favorable effect of consolidated REM sleep and the unfavorable effect of restless REM sleep [1]. That is evidence that the link is not merely a coincidence between two things measured on the same night; the sleep manipulation changed the size of the overnight emotional shift.
Who this pattern matters for, and what is still being tested
The original imaging work spanned volunteers ranging from people with no insomnia complaints to those meeting community-sample criteria for insomnia disorder, so the finding is not confined to a single patient group [1]. The broader idea is that restless REM sleep marks a maladaptive kind of sleep that could be a target for treatment in mental disorders where REM is fragmented [1].
Two lines of work extend the question to specific clinical groups, but with important limits. In borderline personality disorder, a longitudinal study is designed to test whether overnight emotional downregulation relates to restless REM sleep, measured through fragmentation, eye movements, or EEG spectral power, and whether this predicts response to dialectical behavior therapy [4]. That study frames the question rather than settling it. Separately, work on isolated REM sleep behavior disorder uses it as a model to examine overnight habituation of emotional reactivity, including the role of REM phasic events, but it does not address overnight amygdala adaptation directly [2].
An earlier study linked restless REM sleep in insomnia to hyperarousal and to emotional distress that fails to dissolve overnight, which is consistent with the adaptation story but does not itself measure amygdala adaptation [5]. Taken together, the evidence supports a specific, testable claim: fragmented REM sleep is associated with weaker overnight calming of emotional reactivity, and the amygdala is a plausible site of that effect. How strongly this holds across different disorders, and whether repairing REM fragmentation actually restores overnight adaptation, remains an open question that the newer clinical studies are still working through [2][4].
Sources used in this answer
Restless REM Sleep Impedes Overnight Amygdala Adaptation.
Overnight amygdala reactivity decreased in proportion to consolidated REM sleep duration, while restless REM sleep impeded that adaptation, and targeted memory reactivation could enhance both effects.
Isolated REM sleep behavior disorder: a model to assess the overnight habituation of emotional reactivity
Isolated REM sleep behavior disorder is used as a model to assess overnight habituation of emotional reactivity, including REM phasic events, but the study does not address overnight amygdala adaptation.
Selective suppression of rapid eye movement sleep increases next-day negative affect and amygdala responses to social exclusion
Selective suppression of REM sleep increased next-day negative affect and amygdala responses to social exclusion.
R-EMOTION A longitudinal study of the effect of REM sleep fragmentation on the downregulation of emotional distress in Borderline Personality Disorder and its impact on Dialectical Behaviour Therapy treatment
A longitudinal study in borderline personality disorder is designed to test whether overnight emotional downregulation relates to restless REM sleep and predicts dialectical behavior therapy response.
Slow dissolving of emotional distress contributes to hyperarousal
Restless REM sleep in insomnia was linked to hyperarousal and emotional distress that persists overnight, without direct measurement of amygdala adaptation.