Nobody can survive on 90 minutes of sleep per day. Even the Dymaxion schedule, which is basically the harshest you'll find anyone claim to succeed at, is two hours per day (that's 4/3 as long), and nobody
has claimed its success with certainty, especially if you care about it being actually sustainable.
I suspect it's BS. As far as I know that REM sleep is not imprescindible ( nor is NREM unnecessary), so the basis of the theory behind the "overman sleep scheudle" is not too sound. Besides, a while ago I searched for serious coverage of this, and failed to find anything (I admit the search wasn't extremely intensive, though, so feel free to correct me if I'm wrong), whereas I did find somewhere (I think wikipedia?) that while the US army had done some experiments on polyphasic sleep, their goal wasn't to reduce the number of hours asleep, but to divide them, and that the total should still equal the normal ammount. I suspect Overman would just result in being sleep deprived.
Yeah. The harsher polyphasic schedules aren't really shown to be sustainable, or necessarily even achievable, especially not the 90-120 minute ones.
Tip for anyone in this thread, including the OP: You aren't a magic man, a supersoldier, or a comic book character. You aren't going to succeed at feats of psychological/neurological/physical endurance that literally nobody has verifiably pulled off before. Or even the ones extremely
few people have pulled off, considering how well-conditioned you'd have to be.