People suffering with delayed sleep phase disorder (DSPD) generally have normal sleep duration and quality when given ample sleep opportunity, but have sleep onset insomnia and sleep schedules that are severely delayed compared to the general population. Even though it has been shown that alterations in sleep homeostasis may contribute to variation in sleep timing preference, there is now good evidence showing that evening preference is correlated with longer circadian period. An important contributory factor in DSPD is light: excess artificial light at night will delay the circadian clock and reduced exposure to morning light will not help to advance the clock. Importantly, there are significant individual differences in the sensitivity to the effects of melatonin suppression by light, and this raises the important question as to whether the risk of DSPD could also be associated with individual differences in sensitivity to light.