When her daughter was preschool-aged, Rebecca Spencer experienced something familiar to many parents and childminders: the power of a nap. Without it, her daughter would be giddy, grumpy, or both.
Spencer, a neuroscientist focusing on sleep at the University of Massachusetts Amherst, wanted to investigate the science behind this anecdotal experience. “The observation of a lot of people is that a napless kid is emotionally dysregulated,” she says. “So that spurred us to ask this question of, ‘Do naps actually do something to process emotions?’”
Research has already shown that, in general, sleep helps us make sense of emotions. Sleep plays a key role in encoding information based on experiences from the day, making sleep critical for preserving memories. And emotional memories are unique because of the way they activate the amygdala, the brain’s emotional core.
“Amygdala activation is what allows your wedding day and the funeral of your parents to be a day better remembered, more than just any other day of work,” Spencer says.
The amygdala tags these memories as significant, so that during sleep they’re processed for longer and reiterated more than more trivial memories. The upshot is that the memories of emotional significance become easier to retrieve in the future.
But by influencing how memories are processed, sleep can also change the power of a memory itself.
“Sleep is particularly good at transforming emotional memory,” says Elaina Bolinger, who specialises in emotion and sleep at the University of Tuebingen.
In one recent study with eight-to-11-year-olds, Bolinger and colleagues showed children both negative and neutral pictures. The children reported their emotional response using stick figures corresponding to how they felt.
Then some of the children slept. Others did not. The researchers monitored their brain physiology via electrodes from the next room. The next morning, the kids saw the same pictures, plus some new ones. And compared to the children who stayed awake, children who slept were better able to control their emotional responses.
For instance, the sleepers had a smaller emotional response in late positive potential (LPP). Bolinger describes the LPP as voltage measured at the back of the brain. This fires up whenever the brain is processing information – and the spikes are especially large when that information is negative emotion. But humans can control the LPP to some extent. As Bolinger puts it, “We are actively trying to change how we feel about something while we’re perceiving it. So we’re saying, ‘OK, I am trying to not respond very strongly right now, I want to push down my emotional response.’”
This research suggests that sleep helps with both crystallising emotional information – and with controlling how it makes us feel. And this effect works quickly.
“A lot of the research that’s out there right now is pointing in the direction that a single night of sleep is helpful,” says Bolinger. “It’s helpful for processing the memory itself, and it’s also important for emotional regulation in general.”
But not all sleep is created equal.
Types of sleep
Rapid eye movement (REM) sleep is associated with emotional memories, and more REM sleep makes people better at assessing others’ emotional intentions and recalling emotional stories. One theory relates to the absence of the stress hormone noradrenaline during REM sleep. Temporarily relieved of this hormone, the brain may use the time to process memories without the stress.