Renewable Energy at 4 p.m.

06/11/2013 02:31 pm ET | Updated Aug 11, 2013

We were on the 4 p.m. ferry, which departured half an hour late due to delays from boarding the 400-plus passengers returning from a holiday weekend at Cape Cod. A long line formed at the snack bar as soon as we were underway and I noticed that many emerged from the line holding bottles of beer. A very large number of passengers, however, stumbled back to their seats (the boat was bouncing) with napkin-wrapped soft pretzels or bags of mini snack pretzels.

"Looks like everyone needs a carbo fix," I remarked to my companion. "They all look worn out from a day on the beach and fretting over what T-shirt to buy as a souvenir."

Many people, not just the passengers on the ferry, experience an afternoon energy slump. This 4 p.m. energy leakage seems to take over our physical and mental state. Our bodies feel heavy and inert. It is hard to concentrate, to take on new tasks, to be patient and upbeat. The energy drain is worse in the winter when 4 p.m. means dusk or even darkness. But even in the brightness of a late spring afternoon, we may feel less active than we should with the sun still high.

What is curious about this energy drain is that it is rarely physical, even though it feels that way. Indeed, and this may be hard to believe, exercise, simply moving, is an excellent way to restore energy. When people engage even in trivial physical activity, they are energized. During a ferry trip last summer, the pilot announced that there was a whale and its pup on the starboard side of the boat. Passengers who had been slumped in a semi-stupor popped up from their seats and raced over to the right side of the ferry. Where was their exhaustion? It disappeared.

If muscle fatigue were the cause of late afternoon tiredness, exercise would only make it worse. But the brain, not the muscles, generates the cause of the afternoon fatigue. And one reason we feel more vigorous after exercise is that it increases blood flow through our brains, bringing more oxygen and nutrients.

Perhaps one solution for the afternoon mental slump is recess. If there were some way people could leave their cubicles and conference rooms and retail counters to take a fast walk, or a quick spin on a stationary bike, they would feel renewed and ready to continue to work.

There are two other suggested causes for late afternoon fatigue: lack of caffeine and inactive serotonin.

By mid-to-late afternoon, any caffeine we may have consumed in the morning has long disappeared from our bloodstream. Caffeine is a known stimulant, and like any stimulant we take into our body, when it is no longer active we feel tired. It is possible to recharge our mental energy by consuming more caffeine, but doing so comes with a cost, namely a sleepless night. However, that caffeine therapy works and may be an option for those whose sleep is not later effected It should be noted, by the way, that energy drinks are simply overpackaged flashier versions of highly caffeinated beverages.

The other reason for late afternoon fatigue affects everyone, including caffeine users, alike. The neurotransmitter serotonin seems to become less active in the afternoon and the result is a cluster of changes in mental focus, emotional well-being and appetite. Fatigue, lack of concentration, distractibility, impatience, grumpiness and a need to snack may be experienced. Years ago, at MIT, we found that volunteers needed to eat carbohydrates late in the afternoon and this need was always preceeded with some change in their energy, focus and mood. Further studies revealed that when serotonin became more active, these symptoms abate.

Restoring serotonin activity does not require drugs, herbs or supplements. It requires only eating certain foods that, after they are digested, allow tryptophan to get into the brain. Serotonin is made from tryptophan and as soon as this amino acid gets into the brain, it is converted into more serotonin. Enter: pretzels.

The foods that allow tryptophan to get into the brain are any carbohydrates with the exception of fruit. The best carbohydrates are fat-free because they will be digested rapidly and set off the process that gets tryptophan into the brain cells. (They also have the fewest calories.) Fruit sugar does not promote this process. Not much carbohydrate is needed, about 25-30 grams. Protein-containing foods, although they contain tryptophan, do not allow this amino acid to enter the brain because other amino acids in the protein prevent tryptophan access to the "gateway" into the brain. And unfortunately, beer and other alcoholic beverages don't contain enough carbohydrate to allow new serotonin to be made, although they certainly promote afternoon relaxation.

Why does serotonin activity drop in the late afternoon? In all the decades since we discovered its role in afternoon fatigue, we have never found an answer to this. But then again, there are many changes in our daily rhythms such as body temperature, cortisol levels, sleepiness and even hunger. Who is really hungry at 3 a.m. unless supper was skipped?

Afternoon fatigue is natural. So is the solution, which is eating and exercise. Pretzels and running to watch whales seems like a perfect combination.

All aboard!

For more by Judith J. Wurtman, Ph.D., click here.

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