There are two main types of sleep: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep makes up 75-80% of total sleep and occurs in four stages that progressively slow vital signs and make arousal more difficult. REM sleep recurs every 90 minutes and is when dreams occur alongside low muscle tone and irregular heart and breathing rates. Most people experience 4-6 sleep cycles per night alternating between NREM and REM sleep stages. Factors like age, illness, environment, lifestyle, stress, diet, medications and fatigue can influence a person's sleep and rest patterns.