If you're talking 1000's of years of evolution I think you're mistaken. I believe you'll find that the monophasic pattern of sleep is only a modern occurence.
I don't believe for a second that prehistoric man felt safe enough to sleep from sundown to sunup. We sleep in 90ish minute cycles, just like most animals sleep in cycles. Most animals wake up between their cycles, I believe we have conditioned ourselves out of this natural behaviour.
More recently in preindustrialised europe, a lot of early documentation supports a bi phasic sleep pattern.
"Peasant couples were often too tired after a long day's work to do much more than eat and go to sleep, but they would wake later on to talk and make love"(A. Roger Ekirch (2005), At Day's Close: Night In Times Past, pp. 308-310).
We have the siesta in many cultures even Islam suggests a nap between Dhuhr and Asr prayers in the afternoon.
You're right, things can go wrong if you just dive headfirst into altered sleep patterns without doing your homework first and feeling your way very carefully.