The medieval habit of ‘two sleeps’
I believe this theory is largely thought false nowadays, or phrased too strongly at least.I myself have weird sleep cycles. I prefer to stay up into the night and wake up around 12:00, but I vary my sleeping hours by up to 4 hours or so. I go to sleep when I feel like it, and don't feel much discomfort by falling asleep at different times. It leads to weird situations - especially in winter - where I might not see the sun more than a couple hours in 3 days despite being up and around a lot. I don't experience a lot of 'two sleeps', but they do occasionally happen.