COURSE OF DESYNCHRONIZED CIRCADIAN PERIODICITY IN A WOOD MOUSE (APODEMUS FLAVICOLLIS) IN NORTH-FINLAND

1. A free running circadian periodicity in the locomotive activity of a wood mouse (Apodemus flavicollisMelch.) was recorded 172 km south of the arctic circle for two successive summers during natural day. 2 The sensitivity of the animal to the pacemaker varied with age under the same experimental conditions. The same mouse was desynchronized for 48 days in 1967, in 1968, 77 days. 3. The time of desynchronization coincides exactly with the time during which the midpoint of the sun is not more than 6° below the horizon (Civil Twilight). 4. Despite free running periodicity, day and night were perceived as variations in the conditions of continuous lighting. An activity period including a night was shortened, and lengthened when it included a day. © 1969 Springer-Verlag.