THE BLOG
02/16/2011 08:16 am ET | Updated May 25, 2011

Dropping Serotonin Levels: Why You Crave Carbs Late in the Day

You may think you are the only mom who seems to go a little crazy around four or five p.m. when it is getting dark and it's too cold or icy to go outside and your kids are getting cranky or worse. Join the club of moms who find their sanity slipping a wee bit late every afternoon and resort to the only therapeutic agent they can find: sweet or starchy snacks.

Recently, a freelance writer who works from home while taking care of an infant and toddler told me that she dreads the interval between four and six p.m. " I find myself becoming cranky and impatient and I certainly can't concentrate enough to write even if the kids don't need me for a few minutes," she said. "But what is really dreadful is that all I want to do is eat. I feed the kids around five and plan to wait until my husband comes home later to eat my dinner. What happens instead is that I start to snack while the kids are eating and then polish off their leftovers. I am always eating some cold potatoes, gummy macaroni or soggy crackers. Two hours later I eat a regular supper."

There is something about the late afternoon that changes many moms from rational, in control, effective, energetic people into grumpy, irritable and exhausted beings who want to do nothing more than lie on a couch and watch Dr. Phil. And when two kids with an ever-increasing noise level and the Golden Retriever (and the fur he shed yesterday) occupy the couch, you may find yourself going to the kitchen and stuffing yourself with any sweet or starchy snack you can find. It seems a perfectly natural response and one that prevents you from banging your head against the wall.

But consider this. This same stress, kids, dog, and dog fur that sends you chasing after a snack most likely would not have the same affect on you earlier in the day. The "perfect stress storm" that wipes away your control over eating is a combination of time of day and changes in brain serotonin.

Serotonin, "the keeping you in a sane mood" brain chemical, seems to become less active late in the afternoon. This drop in activity is not restricted to mothers with children under 10 and a shedding dog or, for that matter, only to women. Many people experience a decline in their mood late in the afternoon and with it, a need to snack on a sweet or starchy food. The combination of a mood and appetite change is a signal that serotonin activity may be dropping.

No one has ever directly measured this alteration in serotonin activity; doing so would require seriously invasive procedures. But most human research involving serotonin with behavioral measurements can tell us whether there is a change going on in the brain. Some signs of decreased serotonin include grumpiness, irritability, impatience, fatigue, inability to focus, depression and anger. Often these mood changes are accompanied by a craving for carbohydrates.

We discovered this decades ago while carrying out a study at MIT with volunteers who snacked regularly on carbohydrates in the afternoon. Many of them mentioned a predictable change in their mood starting around three-thirty or four p.m. and lasting until five or five-thirty. A salesman told us that he never made cold calls in the late afternoon because he could not tolerate people hanging up on him. This didn't bother him earlier in the day. A competitive bridge player mentioned that time of day as a bad one for focus, and a medical epidemiologist said that she couldn't concentrate on her reports at that time of day. All of them would automatically stop what they were doing to eat a carbohydrate snack like graham crackers or pretzels or popcorn. 20 to 30 minutes later, they were able to resume what they had been doing.

Why does change in mood happen? And why does eating carbohydrate make a difference Eating any carbohydrate (except that in fruit) increases the brain's production of serotonin. So if low or less than fully active serotonin makes moms or anyone else feel grumpy, tired and depressed, eating a small amount of carbohydrate replaces those moods with better ones and does so naturally.

We tested this by measuring the moods of afternoon snackers after they consumed a drink containing carbohydrate or protein. (Protein prevents serotonin from being made.) No improvement in mood was found after they had the protein drink. If they were grumpy before, they were grumpy afterward. But the carbohydrate drink really picked up their mood. They were less depressed, angry, irritable and tired. It is not known why the late afternoon seems to affect serotonin and mood. Is it loss of sunlight? Perhaps, but many people crave carbohydrates at four p.m. in July or August when it is bright and sunny outside.

It is known that women's brains have less serotonin than those of men. Does this mean they are more likely to reach for a snack in the afternoon? No one has ever done a study on what daddies snack on in the late afternoon when they are stuck in the house for hours with yelling kids and a barking dog.

Fortunately for everyone, serotonin mood relief does not require eating much carbohydrate. As long as the snack contains about 30 grams of sugar or starchy carbohydrate, no more than one to two grams of fat (fat slows down digestion and adds unneeded calories) and the same small amount of protein, serotonin can be produced within 20 to 30 minutes. Breakfast cereal eaten dry can make a great snack. Share it with your kids. Their mood will get better as well. (But the dog will continue to shed.)