Plasma melatonin profiles of Romney Marsh sheep in natural photoperiod and in acutely extended darkness.

Plasma melatonin was measured at the summer and winter solstices and the autumn and spring equinoxes in Romney Marsh sheep held under natural conditions in South Australia (35 degrees S). The amount of melatonin detected was generally related to the extent of natural darkness, though the melatonin onset was particularly delayed after dusk in winter compared with other seasons. The duration of detectable melatonin was shorter in summer than at any other season. After each initial 24 h sampling, the sheep were resampled for a further 24 h in acutely extended darkness to mark the phase and duration of suprachiasmatic nuclei activity which is believed to be the source of the melatonin signal. The onset of high plasma melatonin was earlier than the time of natural sunset in spring and summer, but not different from the time of natural sunset in autumn and winter. The offset of high plasma melatonin was later than the time of natural sunrise at all times of year and particularly so in summer. Under the extended dark conditions, the duration of detectable melatonin was longer than that under natural photoperiod at all seasons of the year and the duration of melatonin was again shorter in summer than winter. If melatonin measurements under the conditions of extended darkness do reflect the phase and duration of suprachiasmatic nuclei function then the natural light of the photoperiod can, particularly during long photoperiod conditions, mask the expression of the pacemaker. The findings may have implications for the timing of the breeding season in Romney Marsh sheep.