What happens to melatonin and the circadian rythm when one adopts biphasic sleep cycle?

Question

I have read that the sleep cycle is driven by melatonin, and that there is a natural 24h "sine-wave" of melatonin, where during the day, melatonin is at low levels, while during the night, it is at high levels.

However, I'm wondering, if one adopts a biphasic sleep cycle, where one e.g. sleeps 4 hours during the night and 4 hours during the afternoon, or 6 hours during the night and 2 hours during the afternoon, will that turn the circadian rythm/melatonin-sine-wave into a "double-frequency sine wave"? i.e. will there be a bumb in melatonin during your afternoon nap?

Answer 1

Yes, something quite similar to what you hypothesize in fact!

According to this study (unfortunately, behind a paywall): In short photoperiods, human sleep is biphasic. (Literally the title of the research paper.)

When normal individuals were transferred from a conventional 16-h photoperiod to an experimental 10-h photo-period, their sleep episodes expanded and usually divided into two symmetrical bouts, several hours in duration, with a 1-3 h waking interval between them. The durations of nocturnal melatonin secretion and of the nocturnal phase of rising sleepiness (measured in a constant routine protocol) also expanded, indicating that the timing of internal processes that control sleep and melatonin, such as circadian rhythms, had been modified by the change in photoperiod.

WEHR, T. A. (1992), In short photoperiods, human sleep is biphasic. Journal of Sleep Research, 1: 103–107. doi:10.1111/j.1365-2869.1992.tb00019.x