Adults generally need about 10% to 25% of their total sleep time to be deep sleep, which translates to roughly 40 to 120 minutes (about 1.5 to 2 hours) per night depending on total sleep duration. For example, if an adult sleeps 7 to 8 hours, deep sleep should ideally be around 42 to 96 minutes. This amount can vary by age, health, and individual differences, and tends to decrease with age. Deep sleep is crucial for physical restoration, muscle repair, and overall health.