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The findings support the phase-shift hypothesis for SAD, as well as suggest …

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Materials and Methods
- The circadian basis of winter depression

For more details on experimental techniques used in this study,see Supporting Materials and Methods and Fig. 7, which are publishedas
supporting information on the PNAS web site. Plasma DLMOassessments (see also Fig. 1) and reliably documented sleeptimes were done on what turned out to be a total of 68 SAD patients(see below) before and after 3 weeks of taking melatonin orplacebo capsules. Depression ratings were done weekly by usingthe SIGH-SAD (33). Subjects were randomly assigned to morningor afternoon/evening melatonin treatment or to placebo. Sevento eight capsules per day were taken every 2 h, to produce physiologicallevels of exogenous melatonin to extend the endogenous melatoninprofile either earlier or later, to cause either a phase advanceor a phase delay, respectively, in the endogenous circadianpacemaker (as marked by a shift to an earlier or later time,respectively, in the DLMO) with respect to sleep times thatwere held constant.

At least 27 subjects met established SAD screening criteriaeach year and engaged in 4 weeks of sleeping at home at negotiatedsleep onset and offset times (with the first cohort of 9 subjectsstarting the first or second week of January, and cohorts 2and 3 starting 2 and 4 weeks later, respectively). Sleep timeswere based on self-selected weekday schedules that were heldconstant throughout the protocol and monitored by daily diaries(and, for years 2–4, more reliably by wrist actigraphy).Depression severity (SIGH-SAD score) was assessed weekly, andDLMOs were obtained at the end of the baseline week and after3 weeks of taking 7–8 capsules per day (one every 2 hbeginning at waketime and ending 4 or 2 h before sleep, respectively).Capsules contained placebo or melatonin [0.075 or 0.1 mg (totaling0.225 or 0.3 mg per day), depending on the year of the study]either in the morning (AM) or in the afternoon/evening (PM),or placebo at all times.

Statistical significance is reported above only if linear regressionshad a P value of at least 0.05, by using Pearson's r, confirmedby rank-order Kendall's {tau} and Spearman's {rho} tests, but only Pearson'sis reported, unless otherwise specified. A parabolic curve wasconsidered statistically significant only if it was also significantwhen plotted linearly as a function of the absolute deviationfrom the parabolic minimum. Means are followed by ±SEMs.For comparisons of means between different groups, we used themore conservative Welch two-sample t test, which does not assumeequal variances. Data from the first year of the study wereexcluded in the following analyses, because of the absence ofactigraphic recordings of sleep times. Of the remaining 81 subjects,complete data sets were available for 69, of whom 66 were femalesand the average age was 39 ± 1.1 (range: 23–59),consistent with the known demographics of SAD (5, 6); the outlier(see Fig. 2) was removed, leaving a sample size of 68. Therewere no significant differences between treatment groups (placebo:n = 25; AM = PM = 22) in either (age and gender) demographicor baseline (SIGH-SAD and waketime) variables.

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