[spc476]

For people who light affects their sleep fall generally fall into two camps: those that are oversensitive to light, and those that are undersensitive to light. For the former, (as an extreme example) a flashlight might prevent them from feeling tired (since their body feels like it's day time); whereas for the later, you can shine a tungsten movie light on them and they won't wake up (I think I'm the later---I can easily sleep during the day).