In the following sentence from paragraph 10, what does each…

In the fоllоwing sentence frоm pаrаgrаph 10, what does each refer to? " Each is linked to specific brain waves and neuronal activity." 10. There are two basic types of sleep:  rapid eye movement (REM) sleep and non-REM sleep (which has three different stages).  Each is linked to specific brain waves and neuronal activity.  You cycle through all stages of non-REM and REM sleep several times during a typical night, with increasingly longer, deeper REM periods occurring toward morning.  [BLANK-1]