How might lack of light cause depression?

People troubled by depression usually experience their dark moods in an on-again, off-again fashion.

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