Patients with bipolar disorder (BD) experience persistent circadian rhythm and sleep abnormalities during periods of remission, and biological studies have shown that these patients have abnormal melatonin secretion profiles or reactivity to light. We previously reported the association with BD of a common polymorphism (rs4446909) of the promoter of the acetylserotonin O-methyltransferase (ASMT) gene, encoding one of the two enzymes involved in melatonin biosynthesis. This variant was associated with weaker transcription and lower levels of ASMT activity in lymphoblastoid cell lines. Actigraphy, based on the use of a mobile portable device for the analysis of sleep/wake cycles in natural conditions, may be useful for studies of carriers of the at-risk allele. We studied the association between the ASMT rs4446909 variant and sleep/activity, as assessed with the Pittsburgh Sleep Quality Index (PSQI) and by actigraphy, in 53 subjects (25 patients with BD in remission and 28 healthy controls). The two groups were similar for age, sex ratio, current mood symptoms, body mass index and risk of sleep apnea syndrome. In the total sample, the GG at-risk genotype was associated with longer sleep duration (P = 0.03), greater activity in active periods of sleep (P = 0.015) and greater interday stability (P = 0.003). These associations remained significant when disease status was included in the model. Only the association with interday stability remained significant after correction for multiple testing. This pilot study thus shows that a BD-associated functional variant involved in the melatonin synthesis pathway influences sleep and circadian rhythms in bipolar patients in remission and controls.