Just keep in mind that nothing is exact. That "1.5 hours" is approximate, and will vary between people, and will probably vary from night to night depending on

Just keep in mind that nothing is exact. That "1.5 hours" is approximate, and will vary between people, and will probably vary from night to night depending on all sorts of things you'd never think of, and maybe between the start and end of the night. Plus I'm pretty sure switching from "awake" to "asleep" isn't instantaneous, so you have to count that time as well.