Cores are long blocks of sleep that contain at least one full sleep cycle. They are typically scheduled to be a multiple of a 90 minutes, which is a common [[Wikipedia:sleep cycle|sleep cycle]] length. However, alternative core lengths can be done to either to plan for alternative cycle lengths or in an effort to gain extra time in SWS or REM. Cores are generally considered to be essential, because they provide the necessary amount of SWS to sustain life. As such, schedules without cores (i.e. [[Special:MyLanguage/Nap only|nap only schedules]]) have extremely low success rates.
主段是指至少包含一個完整的睡眠週期的長睡眠塊。它們通常被設置為90分鐘的倍數，即使常見的睡眠週期長度。然而，其他主段長度可以用來配合別的周期長度或加設獲得額外SWS或REM的時間。因為主段睡眠提供維持生命所需的SWS，它們通常被認為是不可或缺的。因此，沒有主段的時程（即僅小睡時程）的成功率極低。
