Rather than explain the 90-minute thing in my own words, I'll quote from a quote in the Glen Rhodes article
If we were to sleep completely naturally, with no alarm clocks or other sleep disturbances, we would wake up, on the average, after a multiple of 90 minutes--for example, after 4 1/2 hours, 6 hours, 7 1/2 hours, or 9 hours, but not after 7 or 8 hours, which are not multiples of 90 minutes. In the period between cycles we are not actually sleeping: it is a sort of twilight zone from which, if we are not disturbed (by light, cold, a full bladder, noise), we move into another 90-minute cycle. A person who sleeps only four cycles (6 hours) will feel more rested than someone who has slept for 8 to 10 hours but who has not been allowed to complete any one cycle because of being awakened before it was completed...
That's why I would say monophasic and biphasic sleepers should always use multiples of 90 minutes for their sleep. For very short naps, there's an exception: if you get 30 minutes or less of sleep, your body will not have time to get to the heavier stages of sleep (it will remain in Stages 1 and 2) and you won't have much trouble waking up. But in a schedule with three or less sleep blocks, I wouldn't use very short naps as part of a regular sleep schedule; they're good at staving off sleepiness temporarily but bad at dealing with overall sleep deprivation (very short naps work much differently in ultrashort polyphasic sleep schedules--like Steve's polyphasic sleep schedule--but only after adaptation is complete).
A tip that I picked up from practical experience and forgot to put down here: incorporate a 15- to 30-minute "winding down" time before each of the two periods of sleep where you lay in bed and do something low-key, such as reading or listening to relaxing music, to make it easy for you to fall asleep at the right time. On the off-chance that you fall asleep early, if your sleep periods are scheduled at multiples of 90 minutes, you'll still wake up in the early part of the sleep cycle and not have to deal with the tiredness associated with waking up from the deep stages of sleep.