Core SleepThe term core sleep literally means the period of time when one is longest asleep on a daily schedule. Most of the current US population has only one time they sleep each day: at night. This is their core sleep time. In multiple other cultures, both throughout history and around the world, it is normal to be biphasic, that is take a "nap" each day at the same time. The non-nap time, again usually at night, would still be the time for core sleep.
Most people do not think about having "core sleep", they just call it sleep. But, for polyphasic sleepers who take naps throughout the day, a "core sleep" means that there is one sleeping time that is longer than others. Someone on a true Uberman schedule would not have a daily core sleep period. There is no time each day when they sleep more than other times.
On the other hand, someone on an Everyman sleep cycle does have a core sleep cycle. In Everyman, there is a period every night when one sleeps for a few hours and then they take a few naps throughout the day. The core sleep is that time, again usually at night, when one would sleep for a few hours.
Some people on an Uberman or Dymaxien schedule will take a day off every so often, usually once every two, three, four, eight, or thirteen weeks. This is technically a longer than normal sleep period, but it is not refered to as core sleep becuase it is not a daily occurence.
There are a couple purposes for adding core sleep to your sleeping routine. The first is to be more like everyone else. The second is to get the other phases of sleep. Why would you want to be more like everyone else? One reason some people appreciate being like others is the psychological and physiological normalcy of having a night time sleep. But, the biggest way in which people on a polyphasic schedule want to be like everyone else is in the flexibility of their sleep time. People on an Everyman schedule, in general, have reported having significantly more flexibility in their schedule than those on Uberman or Dymanian. This has been attributed to the core sleep period, though there really should be more research on this subject before a difinitive conclusion can be drawn.
The second purpose is for people who are looking to be on a polyphasic schedule long term. As noted, some people on an uberman schedule will take a "Day Off". The reason for this is to "catch up" on the other phases of sleep. The core sleep time fulfills the same function on a daily basis. There has been vary little research by Sleep Experts as to the long term effect of polyphasic sleep. In general this is because most sleep experts just dismiss it outright. Some of them have gone so far as to write or say both directly and indirectly that polyphasic sleep is impossible, won't work, and anyone saying that it does work is lying. Obviously, people that have been living a polyphasic schedule would feel that those "experts" are close minded and wrong. But, regardless, there is evidence that all of the phases of sleep do serve a purpose, and since most people don't feel so comfortable taking a chance on shorting their life expectancy, they often opt for an Everyman schedule for long term polyphasic sleeping. Adding a core sleep period adds the other phases of sleep which may indeed be very important to one's health and development.
As such, it is our general recommendation that if one is planning on transitioning to a polyphasic schedule, they start with adapting to Uberman, but plan on eventually moving to an Everyman schedule with a core sleep. This makes polyphasic sleeping more adaptable, "less weird", and eliminates the possibility of long term damage from a lack of deep sleep.