In that respect, seasonal affective disorder (SAD) differs only in that the oscillations follow a seasonal schedule, with the depression usually starting in the fall and lasting through the spring. Lack of light is often blamed for SAD, but just how darker days cause depression in SAD sufferers is still in question, reports the January 2008 issue of the Harvard Health Letter .

Experts debate whether it has been proved that lack of sunlight in winter triggers SAD, but there's certainly circumstantial evidence to support the connection. How might lack of light cause depression? The Harvard Health Letter discusses three theories:

1. The root cause may be insensitivity to light. Most of us go through winter on a relatively even keel because exposure to indoor lighting helps offset the lack of natural light, but indoor light may be too weak for SAD sufferers.

2. There are neural pathways from the eyes' retinas to parts of the brain that help put many of our physiological processes on a 24-hour cycle. Lack of light may put people with SAD out of phase with their biological clocks: awake and active when their internal timers want them snug in bed.

3. A lack of light, or insensitivity to it, may disrupt brain processes influenced by serotonin and dopamine, brain chemicals that play a role in mood.

Light therapy, which involves sitting in front of a bright light for a short time each day, helps some people who suffer from SAD. But antidepressant medications may work just as well, says the Harvard Health Letter .

Also in this issue:

Treating the common cold Illegal use of human growth hormone Types of vitamin E Treating Bell's palsy Low-carb diet and mood By the way, doctor: Warfarin after illness

health.harvard/

Still, some exercise regimens were better than others. Those who exercised the least, walking about 11 miles per week, gained significant benefit, while those who exercised the most, jogging about 17 miles per week, gained slightly more benefit in terms of lowered MetS scores.

One group puzzled the researchers, however. Those who did a short period of very vigorous exercise didn't improve their MetS scores as much as those who performed less intense exercise a longer period.

Kraus said there may be more value in doing moderate intensity exercise every day rather than more intense activity just a few days a week.

In all three of the study's exercise groups, waistlines got smaller over the 8-month period. In general, men who exercised saw greater improvement in their MetS risk factors than women. But Johnson points out that at baseline, the men generally had worse scores than women, so they had more room to improve, she said.

Over the course of the STRRIDE study, the inactive control group “ those who didn't change their diet or activity level at all “ gained an average of about one pound and a half-inch around the waist. That may not sound like much, but that's just six months," Kraus said. "Over a decade, that's an additional 20 pounds and 10 inches at the beltline.

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