With all the recent interest in polyphasic sleep, polynapping etc. I found this article about sleep phases very interesting. It talks - amongst other things - about how it is possible for a human to suffer from either a longer or shorter inner-phase than 24 hours. Having an inner clock set to something considerably different from the normal day-cycle (23- or 25+ hours) can result in a not easily resettable inner clock. That would mean your optimzm period of sleep drifts in one direction. The description for "Delayed Sleep Phase Syndrome" (DSPS) sounds very familliar to me...
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alles Bild, Text und Tonmaterial ist © Martin Spernau, Verwendung und Reproduktion erfordert die Zustimmung des Authors