Each stage of sleep has its own physiological, neurological and psychological features and they flow together to form sleep cycles. All stages of sleep are important. It is the balance of time spent in each sleep stage that is critical to feeling fully rested and refreshed, and to having the mental and physical energy to meet the requirements of the waking day. Together, each stage forms a 90-minute sleep cycle. To perform optimally, we need about 5 cycles per night (90 x 5 = 450 minutes, or 7.5 hours).