The phase of clock gene expression in leukocytes, assessed in the absence of the masking effects of light-dark and sleep-wake cycles, correlates with habitual sleep timing, according to a study published in the May 1 issue of the journal SLEEP.The study, authored by Simon N. Archer, PhD, of the University of Surrey in Guildford, England, and colleagues, focused on 24 healthy subjects in a home environment and clinical research center setting.