Given the above discussion of entrainment of internal pacemakers to external circadian rhythms, it is not surprising that researchers have recently turned their attention to the more subtle and gradual seasonal rhythms, as Hippocrates did 2000 years earlier (Jackson, 1986). It may seem, therefore, that there’s really nothing new under the sun; however, that is decidedly not the case! In fact, as we shall see, highly effective treatment is now available.
Seasonal variations in numerous illnesses occur, including but not limited to the psychiatric disorders of depression, mania, panic, obsessive compulsive disorder, and bulimia. In addition to this list, I have treated four cases of dissociative disorders, including multiple personality disorder, which demonstrated clear seasonal exacerbation (Clark, Hedaya, & Rosenthal, 1995) of the dissociative disorder.
About 10% of all affective disorders are seasonal, of which there seem to be two types. Type A is the classic fall-low, spring-high pattern; type B is the reverse: spring-low, fall-high. A large number of seasonal affective disorder (SAD) patients are known to be bipolar type I or 11, and this seems to be more common if the patient falls into a type A pattern. The frequency with which SAD patients are bipolar ranges from 49 to 93% (Faedda, 1993). Eighty percent of all SAD patients are female.
Given the entrainment of internal rhythms to environmental cues, it is not surprising that the highest frequency of onset of mania or depression comes at the times of greatest change in the seasons: April and September. Seasonal depression (type A) seems to be primarily associated with low energy, hypersomnia (excessive need for sleep), hyperphagia (excessive appetite, carbohydrate craving, possibly related to abnormalities in serotonin neurons of the brain’s feeding center), and depressed mood. In clear-cut SAD, the depressed mood may be secondary to the frustration and helplessness these patients experience around a profound lack of energy and inability to carry on with their lives. Very little research has been done on type B SAD, although some researchers speculate that temperature may be the primary factor in these cases.
Treatment of type A SAD involves phototherapy, which requires that the patient be exposed to full spectrum lights (10,000 lux, no ultraviolet) for a specific amount of time, and at a, certain distance from the light source (usually a light box). The timing of the exposure to the light seems to have little impact on efficacy, as opposed to the importance of duration of exposure. Treatment frequently works within 5 days, and the only side effects seem to be possible eyestrain and irritability (too much light). Recent research seems to indicate that certain drugs that suppress melatonin secretion, such as Inderal (propranolol), are also effective in SAD. Type B SAD is currently treated as other affective disorders are, although further research will undoubtedly change this approach.
The Mechanism of Seasonal Rhythms
No consensus has been reached about the mechanism of light therapy, and there are at least eight hypotheses according to Wirz-Justice (1995). SAD may be related to the length of the photoperiod, the total number of photons taken in per day, melatonin secretion, instability of the circadian rhythms in conditions of extreme daylight or darkness, retinal deficiencies, abnormalities of alpha-2 and/or serotonin receptors in the paraventricular nucleus (PVN), which controls food selection, etc. It seems certain that the simplified circuit diagrammed in figure 2.5, as well as its branches, are involved in the abnormalities of diminished arousal and excessive carbohydrate intake, and that serotonin dysregulation is involved as well.