Background: Dim Light Melatonin Onset (DLMO) can be calculated within a 5-point partial melatonin curve in saliva collected at home. We retrospectively analyzed the patient melatonin measurements sample size of the year 2008 to evaluate these DLMO calculations and studied the correlation between diary or polysomnography (PSG) sleep onset and DLMO. Methods: Patients completed an online questionnaire. If this questionnaire pointed to a possible Delayed Sleep Phase Disorder (DSPD), saliva collection devices were sent to the patient. Collection occurred at 5 consecutive hours. Melatonin concentration was measured with a radioimmunoassay and DLMO was defined as the time at which the melatonin concentration in saliva reaches 4 pg/mL. Sleep onset time was retrieved from an online one-week sleep diary and/or one-night PSG. Results: A total of 1848 diagnostic 5-point curves were obtained. DLMO could be determined in 76.2% (n = 1408). DLMO significantly differed between different age groups and increased with age. Pearson correlations (r) between DLMO and sleep onset measured with PSG or with a diary were 0.514 (p = <0.001, n = 54) and 0.653 (p = 0.002, n = 20) respectively. Conclusion: DLMO can be reliably measured in saliva that is conveniently collected at home. DLMO correlates moderately with sleep onset. (C) 2011 Elsevier B.V. All rights reserved.