Absence epilepsy is characterized by disturbed consciousness and generalized, synchronous, bilateral, 3–4 Hz (in humans) and 7–11 (in rats) spike-wave discharges (SWD) in EEG. It has been documented that the occurrence of absence seizures shows circadian pattern both in humans and animals. WAG/Rij rats, a well known, validated animal model of human absence epilepsy, show a clear circadian distribution of SWD (maximum in early hours of dark period, minimum after the onset of light). Moreover, a strong correlation between the occurrence of SWD and the level of vigilance exists: SWD are less prone to occur during active wakefulness. SWD rhythm and its relationship with activity in entrained and constant condition were investigated. Chronic EEG and general activity recordings were made in six adult WAG/Rij rats. Animals were kept in 12:12 light–dark cycle. The light regime was changed after 10 days into constant dim light (<6 lux) in which rats were maintained for the following 20 days. The period lengths of both rhythms were estimated by the Cosinor method. Clear 24 h rhythms of activity and SWD were found under entrained conditions, in constant condition both rhythms were free-running. Periods’ length of activity was increased while period of SWD rhythm was changed differently for different animals. Observed rhythms’ splitting suggests decoupling of these two rhythms in constant condition and that the SWD rhythm is no longer controlled by a master clock.