Random
From Polyphasic Sleep Wiki
Random sleep is irregular sleep, which has no structure, changes from day to day and can't be categorized it into any current specific sleep patterns.
Mechanism
Random is a highly popular sleep pattern, which has plenty of variants. Since it inherently has no rules, such schedule would fit any lifestyle. Sleep may be taken whenever possible or tired enough.
One of the characteristics of Random sleep is the level of randomness, which may be higher or lower.