Sleep cycles are the natural patterns of sleep that our bodies go through each night. A complete sleep cycle typically lasts about 90 minutes, and during a normal night's sleep, you'll cycle through 4-6 complete cycles. Each cycle consists of different stages, from light sleep to deep sleep and REM (Rapid Eye Movement) sleep.
Waking up during the right phase of your sleep cycle can make a huge difference in how you feel. If you wake up during deep sleep, you're likely to feel groggy and disoriented - this is called 'sleep inertia'. However, if you wake up during light sleep, you'll feel more refreshed and alert. This is why timing your sleep in 90-minute cycles can help you wake up feeling better.
While individual needs vary, most adults need between 7-9 hours of sleep per night, which corresponds to about 5-6 complete sleep cycles. The quality of sleep is just as important as the quantity - completing full sleep cycles leads to more restorative sleep.