When her daughter was in preschool, Rebecca Spencer experienced something that many parents and caregivers are familiar with: the power of a nap.
Without a nap, her daughter was dizzy, moody, or both at the same time.
Spencer, a sleep-related neuroscientist at the University of Massachusetts Amherst in the United States, wanted to investigate what lies behind this anecdotal experience.
"Many people realize that a child without a nap is emotionally dysregulated," he explains. "This has led us to ask ourselves a question: Do naps really help to process emotions?"
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Scientific research has already shown that, in general, sleep helps us understand emotions. In fact, it plays a key role in coding information extracted from the day's experiences, so preserving memories is essential.
And the emotional memories are unique because of the way they activate the body of the amygdala: the emotional core of the brain.
"Activating the amygdala's body is what allows you to remember your wedding day and the funeral of your parents more than any other day at work," says Spencer.
The body of the amygdala labels these memories as meaningful, so that during sleep they are processed longer and are repeated more than other trivial memories.
The result is that emotionally important memories are easier to retrieve in the future.
But by having an influence on how memories are processed, the dream can also change the power they have.
"Sleep is particularly effective when it comes to transforming emotional memory," says Elaina Bolinger, an expert on emotion and sleep at the University of Tuebingen in Germany.
In a recent study, Bolinger and his colleagues showed negative and neutral images for children between 8 and 11 years of age. The children showed their emotional response by choosing simple designs of people.
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Later, some children slept and some did not. The researchers controlled their brain physiology through electrodes from the next room.
The next morning, the children saw the same images as well as some new ones. And in comparison to the children who stayed awake, the children who slept better controlled their emotional responses.
This research suggests that sleep helps to crystallize emotional information and control how it makes us feel. And this effect happens quickly.
"Much of the current research indicates that a single night's sleep is already helpful," says Bolinger. "It is useful for processing memories, and it is also important for emotional regulation in general."
But not all the dream is the same.
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Rapid eye movement (REM) sleep is associated with emotional memories, and having more REM sleep causes people to better evaluate others' intentions and remember emotional stories.
One theory points to the absence of noradrenaline stress hormone during REM sleep. Temporarily released from this hormone, the brain can process memories without stress.
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Simon Durrant, head of the Sleep and Cognition Laboratory at the University of Lincoln in England, highlights another aspect.
The prefrontal cortex is the most developed part of the brain: it is where Durrant says: "the human impulse to remain calm and not react immediately to things."
During wakefulness, this is the part that keeps the body of the amygdala under control and therefore the emotions. During sleep, this connection is reduced.
"In a sense, during REM sleep, the emotion is excessive."
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But Spencer believes that non-REM sleep also plays an important role. Slow-wave sleep (SWS) is the first phase of sleep that consolidates memories and is especially effective in processing neutral memories.
Spencer's research suggests that the amount of SWS activity during sleep affects the way emotional memories are transformed.
Naps mainly consist of non-REM sleep. And a recent article co-authored by Spencer seems to be the first to show that naps, not just nocturnal sleep, contribute to the processing of emotional memory in children.
Without slumbering, the children showed a tendency towards emotionality. With snooze, they responded similarly to neutral stimuli and emotional stimuli.
In short, he assures us that "if they do not sleep, children become hypersensitive to emotional stimuli" because they did not consolidate the emotional baggage of that day.
Spencer believes that naps also contribute to emotional processing in adults, though not in the same proportion. An adult has a more mature hippocampus and thus more ability to preserve memories. Not sleeping does not hurt them that much.
However, this is only to a certain extent. Spencer's research on aging suggests that "we need to consolidate memories more often as we age."
Interestingly, older adults show a tendency towards positive memories, while young adults tend to negative ones.
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This may be because children and adolescents focus on negative experiences because they contain important information to be learned: from the dangers of fire to the risks of accepting a drink from a stranger.
But at the end of life, people prioritize the positive. They also have less REM sleep, the kind of sleep that will probably save negative memories, especially in people with depression.
Sleep researchers are also analyzing the potential of certain facets of sleep to treat post-traumatic stress disorder (PTSD).
One study suggests that sleeping within 24 hours of a traumatic experience makes these memories less distressing in later days. For people with anxiety, sleep therapy can help them remember that they have eliminated fear.
On the contrary, wake therapy – in which people are deliberately deprived of sleep – is spreading as a method for treating depression.
Insomnia, in some cases, can have a protective effect. Spencer notes that, following a trauma, "the natural biological response under such conditions is to have insomnia."
Thus, sometimes it may be good that lack of REM sleep impairs the brain's ability to consolidate emotional memories.
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"There is evidence that people with higher REM sleep tend to be more depressed," says Durrant. The specialist believes that this is because a subgroup of people with depression once again consolidates negative memories during REM sleep.
"I do not think I'll be able to see this problem solved," he says of all potential clinical applications of sleep and wake therapy.
But what is clear is that certain types of decision-making improve after sleep, in part because of the way sleep regulates the whole swirl of feelings.
Bolinger explains this clearly: in general, "sleep helps you feel better".
In the end, the best recipe for a broken heart or a fuzzy mind can be a nap.
Read the original story in English on BBC Future.