49-55 The success of the chaos theory seems to be, in my impression, due to epistemology: the fact that a phenomenon obeying deterministic laws could be unpredictable can be seen as a sign

of the defeat of the causality principle. In several cases, this conclusion seems to apply to chronobiology.
Winter depression (seasonal affective disorder, or SAD) has proved to be a useful model for evaluating the role of circadian rhythms in psychiatric and sleep find more disorders. The successful treatment Inhibitors,research,lifescience,medical of the first patient,1 as well as the first controlled study using bright light,2 assumed SAD to be a disorder of seasonal biological rhythms. Both studies were based on the finding that bright light could suppress melatonin production in humans.1 Accordingly, bright light exposure was scheduled in the morning and late afternoon/evening in order to mimic a spring photoperiod. The investigators involved in these early studies diverged into two groups: our group focused on a circadian approach to SAD3 while the other group did not.4,5 The circadian approach was based on the phase shift hypothesis (PSH) Inhibitors,research,lifescience,medical which states that most patients with SAD become depressed in the winter, at least in part because of a phase delay in circadian rhythms relative to the sleep/wake cycle.6-9 The PSH further postulates that a smaller subgroup of SAD patients becomes depressed in the winter because of a phase advance. Inhibitors,research,lifescience,medical In 1987, based on our hypothesized

phase response curve (PRC) Inhibitors,research,lifescience,medical to light and prior preliminary light studies in humans, we reported that bright light scheduled in the morning causes a phase advance (a shift to an earlier time) and that bright light scheduled in the evening causes a phase delay (a shift to a later time), using the dim light melatonin onset (DLMO, that is, the time of the beginning of melatonin production in dim light) as the marker for circadian phase Inhibitors,research,lifescience,medical position.8 We also reported

that seven of eight SAD patients preferentially responded to the antidepressant effects of morning light, whereas one patient preferentially responded to evening light.8 The combination of morning and evening bright light counteracted each other. There was a statistically significant, but small, delay in the DLMO of the patients compared with the controls at baseline. The clinical recommendations following this report published 20 years ago9 remain state-of-the-art and unchanged, except that light intensity can be increased to 10 000 lux, offering some shortening of minimal Isotretinoin exposure duration. Accordingly, these recommendations are reprinted in Table I SAD patients and controls were phase shifted with bright light according to Figure 1 (which also includes how to use melatonin administration to cause similar phase shifts). Figure 1. Use of bright light and low-dose melatonin to treat circadian phase disorders. Adapted from ref 10: Lewy AJ, Sack RL. The role of melatonin and light in the human circadian system. In: Buijs R, Kalsbeek A, Romijn H, Pennartz C, Mirmiran M, eds.