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Maybe those Insta-worthy #weekend vibes reliably fade come Sunday afternoon. Or your drive to take on the world only flares up once the workday dies down. Mood shifts are a casualty of being human. We all have ups and downs, and a growing body of research suggests that our "ups" have less to do with what we're doing than when we're doing it. Like public transit and broadcast television, good moods appear to operate on a daily schedule. The more we understand our feel-good rhythms, the more we can take advantage of our emotional happy hour. And that's where chronobiology comes in. 

The relatively new field of science concerns the hardwired biological rhythms that control the timing and duration of physiological and mental processes. Chief among these rhythms is the circadian rhythm, which keeps us drumming to the beat of a 24-hour-cycle. Circadian rhythm is best known for its role in regulating sleep-wake schedules. But, a number of other processes work under it, including daily fluctuations in good mood — but not bad mood. In other words, people tend to feel inspired, enthusiastic, cheerful or otherwise positive at predictable times of day. And those times correspond both to the number of hours they’ve been awake and their chronotype, aka whether they’re naturally morning larks or night owls.

Circadian rhythm is best known for its role in regulating sleep-wake schedules. But, a number of other processes work under it, including daily fluctuations in good mood — but not bad mood.

As Brant Hasler, a psychiatrist at the University of Pittsburgh who studies the role of sleep and circadian rhythms in regulating mood and motivation, explained: “There’s something in the positive mood system and the reward system that’s under control of the circadian system."

The Biology of Mood Regulation

Researchers can’t definitively say why circadian rhythms control positive mood only, but a prevailing theory implicates evolution. The psychologist David Watson first identified the phenomenon in the 90s, while administering psychological mood scales to groups of college students at different times of day. He noticed that students reported positive moods at consistent, predictable times each day.

Per Hasler: “Watson’s idea — and this is still sort of my working model — is that there’s something called a 'behavioral activation system,' which motivates us to go out and pursue rewards, and that it’s evolutionarily adaptive to have this system peak at different times of day, so that it will drive us to go out and seek rewards during times of maximal reward and less danger, and keep us sitting tight during times of more danger and less opportunity for reward.”

Let’s unpack that.

Researchers can’t definitively say why circadian rhythms control positive mood only, but a prevailing theory implicates evolution.

Firstly, think of “positive affect” as an umbrella term for “good” mood states that predispose us to seize the day. Research on positive affect echoes advice you might give to a down-in-the-dumps friend: A good attitude goes far. People who are engaged in the world tend to find themselves with second dates, follow-up interviews and so forth.

“There’s longitudinal data showing that when people have more positive affect they do better at job interviews and are more likely to get married," said Hasler. "And it's been done in a way that shows it’s not that getting married makes them happy, but that being happy beforehand makes them more likely to find a partner.”

Bad moods don’t peak and fall according to a daily schedule because a different biological system underlies negative affect.

As for the evolutionary theory? Ancestral humans had to fight tooth, nail and loincloth for every berry and boar they brought back to the cave. To maximize their chance of making it through the day, hunters-and-gatherers needed to outsmart predators, competing clans and various other environmental threats. The frame of mind most likely to facilitate survival was one of strong positive affect. So, according to Watson’s theory, the behavioral activation system evolved to ensure positive affect peaked when pre-humans were awake and likely to venture out into the world. And, positive affect hit its nadir when humans were asleep and least in need of channeling a can-do attitude.

Bad moods don’t peak and fall according to a daily schedule because a different biological system underlies negative affect. Getting agitated at the same time every day wouldn’t have maximized chance of survival.

The Daily Height of Good Vibes

Research (including Hasler’s) places positive affect under circadian control. In one 2008 study, for example, Hasler recorded audio of college students going about their days and identified sound clips of three behaviors indicative of positive affect: laughing, singing/whistling and engaging in social interactions. The behaviors peaked between mid-afternoon and early evening.

Neuroimaging research also suggests there’s something about the afternoon. A 2014 study showed that people who learned they’d won money exhibited heightened activity in the brain’s reward pathways, indicating they felt more jazzed about the prospect of collecting earnings later in the day.

The Chronotype Effect

Daily positive-mood rhythms, however, appear to differ somewhat by chronotype. In terms of timing, Hasler explained, early risers hit peak positivity a few hours earlier than night people do. But, more than that, night owls have comparably lower peaks and valleys — they don't get quite as happy, and sink lower, than their a.m.-loving counterparts.

Night owls have comparably lower peaks and valleys — they don't get quite as happy as their a.m.-loving counterparts. And they sink lower.

Night people's blunted capacity for positive moods isn't surprising. In general, research depicts night owls as getting the short end of the stick. They’re more depressed, addicted, risk-prone and susceptible to cardiovascular disease. Social Jetlag theory explains owls' shortcomings as a product of misalignment: They'd have fewer no-good-very-bad-horrible days if their biology and work (or school) schedules weren't at odds. But Hasler says it’s not clear that theirs is a crisis of bad timing. 

“There’s some data suggesting that it’s not really about their schedules being out of sync with their chronotype, but that, for some reason we don’t yet know, evening types are just worse off, regardless of when they sleep," he said. "But that’s an open question.”

Our Awareness of Mood Swings

People generally know whether they function best before breakfast or come alive in the moonlight. But Hasler doubts many people know when their positive moods peak, or that they peak systematically.

“What’s fascinating to me is that these rhythms are ongoing and outside conscious awareness to some degree,” he said. 

It may be worth paying closer attention. “For morning types, even though they’re not at their peak positive affect in the morning, they may be relatively good at that point, where evening types might be really bad off," Hasler said. "Sometimes it’s not about taking advantage of peak opportunities, but [evening types could] at the least avoid their worst times.”

But, before people start tracking their happiness levels, they should brush up on factors likely to throw off their rhythms. Namely? Sleep deprivation. 

“With a lot of sleep restriction they may have wider swings and lower troughs," Hasler said. "They’d probably still find some rhythm, but it would be different than the norm. The bad times are going to be even more dramatically bad.”