The dark hours of sleep

I’ve learned over the years that there is a timespan of hours that if my sleep falls into that range, I’m going to be a complete waste the next day. That range, 4-6 hours. If I get more or less sleep than 4-6 hours, I’m good to go. But for some reason that range is the death of me. Last night due to a server maintenance that went south, I got 5 hours.