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Feats of willpower, like dieting, quitting smoking, staying sober and ignoring texts from the person saved in your phone as "garbage," are understandably difficult to carry out. And there are a number of well-recognized external and internal factors that affect your ability to just say no, such as easy access to temptation, your emotional state and your fatigue level. It's easier to stick to Gwynneth's diet, for instance, if you stock the fridge with NBK (nothing but kale) instead of cookie butter and queso. And, when you feel sad, stressed, nervous, tired or maybe even bored, you might be especially likely to seek out your vice of choice. But, according to some researchers, these factors are influenced by another, less obvious and largely overlooked component in the self-control equation: time of day.

In a new review paper, Brett Millar, a professor of health psychology and doctoral student at the City University of New York, says we need to understand failures and successes of self-control in the context of when they occur. Mustering the strength to pass up a donut, Millar argues, is a different challenge at 10am vs. 10pm. And we don't all experience the same daily fluctuations in self-control — the time of day at which you're most likely to, say, fish cigarettes out of the trash depends, to some degree, on your chronotype (i.e., being a night owl or a morning lark). 

Self-control is a perennial hot topic in the psychology world. The underlying question in understanding self-control is one of inaction: What determines whether or not you can stop yourself from giving into a temptation or impulse? If we don't take into account individual differences in chronotype (that part comes next), Millar argues that nighttime is the hardest time of day to exercise self-control. 

The more tired, drained, and worn out we are, the more our self-regulation suffers, whether we’re trying to regulate our behaviors, our thoughts, or our emotions.

The crux of Millar's argument is that self-control is both most needed and most stretched-thin at night. It's needed because temptations like alcohol, junk food and anonymous sex are more likely to be in your face after the sun goes down. And it's stretched-thin because you've been awake, and dealing with whatever the world throws at you, all the livelong day.

"For someone who wakes at 7am, [a] period of impairment caused by sleep-deprivation will begin roughly from 9pm onwards," said Millar. "And ‘9pm onwards’ is precisely when many of us are in the very predicaments which require our optimal functioning in order for self-regulation to succeed: staying out at a bar, driving by a fast food restaurant, thumbing through the dessert menu, or making choices towards the end of a romantic date."

So, at night, you're fatigued because it's been awhile since you last slept. But, Millar explains, you're also mentally drained from already having spent the day exercising self-control. This second idea — that regulating thoughts and emotions wears you down — is the crux of ego depletion, a once-beloved theory that, after faring poorly in recent replication studies, has lost some of its luster. According to ego depletion, self-control wears out like a muscle, making each feat of self-control harder than the last one. So, passing up that morning donut makes it harder to skip post-lunch cake, and then even harder to wave away a pina colada during happy hour, and so on. 

"Essentially," said Millar, "the more tired, drained, and worn out we are, the more our self-regulation suffers, whether we’re trying to regulate our behaviors, our thoughts, or our emotions — and researchers need to consider this gradual ‘wear and tear’ across the full span of people’s waking hours."

Ego depletion is the basis for a phenomenon called the morning morality effect, which says people become more likely to cheat, lie and engage in other dishonest behavior later in the day, as their energy levels fall. The morning morality effect bore out in a 2013 study, but follow-up experiments highlighted its limitations — namely, that the effect only emerged in morning people. Night owls, by contrast, showed more ethical rigor later in the day. 

And chronotype may be similarly relevant to daily fluctuations in self-control. At 10pm, owls and larks encounter the same nighttime temptations. In fact, since owls are more likely to be awake late into the night, they probably have more opportunities to break bad after dark. But, because owls operate on a delayed schedule, they may actually be relatively alert at night and thus better equipped to exert self-control at midnight than at dawn.

"In general," Millar wrote, "morning types are expected to have the advantage in self-regulation during the day and begin to experience impairments sooner at night, whereas the self-regulatory capacity in everning types may peak later in the day and evening, and may not dip until later at night and the following morning." 

It's useful to understand when you're most susceptible to breaking a diet or falling off the wagon so you can learn to anticipate moments of weakness.

Although, the relationship between chronotype and time of day is tricky. There are other qualities associated with chronotype, as well as other circadian factors, that could affect how well owls and larks exercise self-control at night. For instance, research has shown that owls tend to live in the moment whereas larks are more future-oriented. This is thought to give larks the advantage in delaying gratification. At night, then, it's conceivable that an owl would live life to the fullest without giving much thought to next-day consequences. A lark, on the other hand, would do the lark thing and avoid making choices that might screw them over tomorrow. 

It's time for the psych field, Millar says, to pay more attention to the role time-of-day and chronotype play in our ability to exercise self-control. Understanding how these three factors interact could improve treatment for addiction as well as for other disorders marked by impairments in self-control, such as insomnia, anxiety and PTSD. 

"Windows of vulnerability, and of resilience, can be targeted by prevention and intervention efforts," said Brant Hasler, whose research at the University of Pittsburgh focuses on the role of sleep and circadian rhythms in regulating mood and motivation. In other words, it's useful to understand when you're most susceptible to breaking a diet or falling off the wagon so you can learn to anticipate moments of weakness and regulate your feelings, thought patterns and behavior accordingly. Additionally, researchers including Hasler have suggested scheduling therapy for the time of day when someone's at their circadian peak, to give them the greatest chance of benefitting from treatment that requires effort and motivation. 

Millar plans to continue exploring the impact of time-of-day on various risky behaviors and hopes to see more treatment efforts that can prepare people for times of day when their capacity for self-control is on "thin ice."