Ninety minutes is the magic number, the length of one adult sleep cycle. One 90-minute nap resets my brain enough to carry on a little longer. Three 90-minute stretches over the course of a night and I am semi-human the next day. Zero such stretches—the nights when Younger Brother wakes at the end of each of his 45-minute sleep cycles—and life is gray and heavy. I fantasize about checking into a hotel, taking a sleeping pill, and crashing for 12 hours beneath a puffy white duvet.
(Update: He slept for five-and-a-half hours straight last night, then went down for another two. It was glorious.)