Short-term temporal cycles in the frequency of suicide. United States, 1972-1978.

Vital statistics data for the United States for the years 1972-1978 have been analyzed for variation in frequency of suicide by month, day of the week, day of the month, and phase of the lunar cycle. Frequency of suicide showed a peak in the spring months, a lesser peak in the fall and was lowest in December. Suicide was most frequent on Mondays and declined during the week to a trough on Saturdays (in the summer) or Sundays (in the winter). There was a previously unreported cycle by day of the month, of comparable magnitude to that seen by month and day of the week; suicides were most frequent around the 5th of the month and least frequent in the last days of the month. February was the only month which failed to exhibit this within-month cycle. There is no satisfactory explanation for any of these temporal cycles. There was no relationship between number of suicides and phases of the lunar cycle.