Summary.1The diurnal rhythm of various activities in three species of sea birds was studied in the island of Jan Mayen (lat. 71oN.) in July and August 1950.2The number of Fulmars flying past a certain point decreased strongly around midnight in July and even more markedly in August.3In Brünnich's Guillemot the number of birds alighting at a colony fluctuated but it was uncertain whether the fluctuations were regular or not.4On the other hand the number of young of the same species leaving their nesting ledges showed a very marked peak during the hours preceding midnight. This peak coincided with a peak at the same place in the numbers of Glaucous Gulls, which preyed on the young guillemots as they flew down to the water.5The amount of sleeping and preening by Kittiwakes at a nesting cliff fluctuated little during the 24 hours in July (when they slept about 7 hours a day) but more in August.6The “ultimate factors” responsible for the rhythms observed in Jan Mayen are discussed.7The duration of sleep is discussed for the birds studied, and compared with that of other spe