Introduction: Self-reported habitual sleep duration has been used widely in epidemiological research yet this measure remains to be validated. We evaluated whether simple sleep duration questions concord with sleep diaries in an online sample. Methods: Australian Adults aged 18+ years completed an internet survey examining measures of sleep, socio-demographic risk factors and a 7-day sleep diary. We examined single question (how many hours of sleep would you normally get?) and two question assessments (difference between sleep and wake times) to a 7-day sleep diary estimation of sleep duration. Using Bland-Altman plots and associated statistics we tested systematic differences, precision, and systematic bias. We also evaluated whether the differences were consistent along the entire range of the measurement and whether they were associated with any socio-demographic risk factors (Spearman's rho). Results: Data were analyzed from 1,662 participants (67.3% female). Bland-Altman plots displayed visual discrepancies between both one question and two question reports of sleep duration compared to sleep diaries. Both the single (-17mins) and double question (8mins) sleep duration estimates differed significantly (both p<.001). These simple estimates only agreed to within +/-2.5-3 hours compared to diary estimates. The measure was also weakly systematically biased (Rho= +0.204 and +0.309, p<.001) through the measurement range. There were significant differences and associations between differences in sleep duration estimation and determinants of health. Conclusions: Simple questions estimating habitual sleep duration are imprecise and systematically biased in a large online survey. The amount of difference is correlated with well-known sociodemographic risk factors.