Can you explain sleep cycles?

Can you explain sleep cycles? 

When we fall asleep at night there are changes in a range of physiological processes. In other words, many things change in our bodies as we fall asleep. For example, there are changes to our brain activity, heart rate, and muscle tone. In addition, there are more than simply two states (wake and sleep). In fact, sleep can be broken down even further. Adults experience non rapid eye movement (NREM) sleep which can be partitioned into N1 (the lightest stage of sleep), N2 (slightly deeper sleep) and N3 (deep sleep). We also experience Rapid Eye Movement (REM) sleep, during which our bodies are paralyzed while our eyes dart about in rapid movement – hence the name “rapid eye movement.” We cycle through these different stages of sleep during the night, moving from NREM to REM sleep. Each cycle typically takes between 90 and 110 minutes in adults.

In contrast to the sleep cycles experienced by adults, they can look rather different in babies. For a start, babies do not experience these different stages of sleep. Instead, when a baby is first born, it is only possible to distinguish what is referred to as ‘active’ and ‘quiet’ sleep. Active sleep is similar to REM sleep, yet babies are twitchy and jerky during this stage. Quiet sleep is similar to NREM sleep – but before a baby is six months old it is not possible to differentiate among N1, N2, and N3 in the same way as is in adults.

Babies’ sleep cycles also differ in other ways. For instance, a baby’s sleep cycle is typically shorter than the sleep cycle of an adult. A premature baby may take just half the time an adult does to complete a sleep cycle (just 45 to 60 minutes). Further, the order of a young baby’s sleep cycle stages is different than the order of sleep cycle stages in an adult. Young babies enter REM-like (active) sleep before NREM-like (quiet sleep), instead of the other way around for adults.

By adolescence, sleep cycles are roughly the same length as those of adults, although the composition of different types of sleep will continue to change throughout the life course – with sleep becoming lighter in older adults.