Documenting very early mood improvement with bright light exposure in SAD patients is consistent with other studies reporting that brief administration of light is clinically active. Rot et al (10
) showed that bright light in contrast to dim light prevents mood lowering, induced by acute tryptophan depletion. Natural bright light in normal subjects for 30 minutes improved one dimension of mood status, ‘pleasantness’, on the Mood Check List 3 (11
). In a study by Goel and Etwaroo (12
), bright light improved total mood disturbance, depression, and/or anger within 15–30 minutes of treatment and this effect was similar to that of auditory stimulus and high-density negative ions.
Improvements in alertness, attention and vigilance, especially during adverse conditions, after brief exposure to light have been reported by several studies. For instance, Babkoff et al (13
) reported that bright light exposure of 1 hour improved subjective alertness and reaction time as compared with dim light. Similar results were found by Wright et al (14
). In healthy volunteers, it was shown that a combination of 20 minutes of afternoon nap and exposure of bright light for 1 minute after the nap significantly improved cognitive performance when compared with caffeine after nap (15
). In another study, improvement in alertness at night time was seen with 90 minutes of bright light as compared with dim light.
What could be the underlying mechanisms involved in the early improvement in depression scores with bright light treatment? Previous chronobiologic mechanisms implicated in response and remission of depression in SAD patients, such as shortening of nocturnal melatonin secretion, prolonged in SAD patients (17
) or phase advancing (18
) would probably have no role in the effect of brief immediate response to light. On the other hand, the previously reported effect of a course of light on neurotransmitters including serotonin (22
) and norepinephrine (22
) could be also involved in the effects of short-term exposure to bright light. Supporting serotonin involvement, a study by Lambert et al (27
), invasively measuring the serotonin turnover, reported that the light intensity on the day of the test rather than average of previous days correlated best with brain serotonin turnover, suggesting a contemporaneous rather than moving-averaged or lagged response of bright light, suggesting an interaction between immediate bright light treatment and serotonin levels. Short exposure to bright light (as compared with dim light) in mildly seasonal healthy young women prevented mood worsening after tryptophan depletion (8
Our study has several limitations. The study was retrospective and the sample size of the patient population was small (15 patients). Our study was conducted in an artificial environment, in a scanner, with invasive catheters, with the presence of raters etc, but it was highly precise. Although we report the effects of short exposures, we were unable to determine how long the immediate mood improvement after light would last. So possibly a longer duration of light, although unnecessary for a greater immediate effect, would nevertheless be better in sustaining that effect. In addition, as we did not have a control, we do not know to what degree the immediate response is a nonspecific effect, based on expectation (placebo) or the close interaction with the patients during the PET scanning procedures. Finally, other cardinal symptoms of SAD, such as increased appetite, carbohydrate craving, and sleepiness were not measured.