When you choose to get up will depend on the constraints of your life, your work and the people in it, and also your genetic chronotype, which is the time of day that you naturally are almost alert and want to sleep. This free chronotype assessment can give you some clues as to yours.

That we have different chronotypes combines with our individual 90 minute sleep cycles to be evolutionarily protective. Together, they mean that, in any group of people, there would always be someone more likely to awake or very lightly asleep, rousable should there be a danger.