Intrinsic circadian period of adolescent humans measured in conditions of forced desynchrony

Circadian timing was assessed with forced desynchrony (FD) in 10 healthy adolescents (five boys, five girls; mean age 13.7 years). Following 10 days of entrainment to a fixed light-dark (LD) schedule at home, participants were studied under dim light (<20 lux) in the laboratory. A 28-h schedule (FD) was imposed for 12 x 28-h cycles. Saliva was collected at 30- or 60-min intervals throughout; core temperature was measured in constant routines (CR) before and after FD. Intrinsic circadian period was estimated by linear regression using temperature minimum from CRs and dim-light salivary melatonin onsets and offsets from FD. Average intrinsic circadian period for core temperature (n = 7) was 24.30 +/- 0.20, for melatonin onset was 24.33 +/- 0.21, and for melatonin offset was 24.35 +/- 0.21. Intrinsic circadian period in every adolescent was greater than 24 h. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.