You are correct. But in attion.... For any given search, there are only so many folks available for a search party. So what are you going to do... Put half on the day shift when they can see well and the chances of rescue are higher and half on the night shift when chances are lower? Of course not... Put them all on the day shift when chances are higher, further boosting your chances of finding. And humans are humans, they can't work 24 hours for multiple days in a row... They gotta sleep. So let them sleep when the chances of discovery are diminished.