Sleep cycle

A sleep cycle is one complete progression through the stages of sleep. Broadly, sleep is divided into two types: active sleep (also called rapid eye movement, or REM sleep), in which the sleeper wakes more easily, and quiet sleep (deep or non-REM sleep), in which the brain rests more deeply. A full night's sleep consists of several such cycles repeated in succession.

How does a baby's sleep cycle differ from an adult's?

According to the Lullaby Trust, a baby's sleep cycle lasts approximately one hour, compared with around 90 minutes for an adult. Babies and adults also enter sleep in opposite order: adults fall quickly into quiet (deep) sleep, which accounts for approximately three quarters (75–80%) of their cycle, before moving into active (REM) sleep. Babies, by contrast, enter active (REM) sleep first — a lighter phase lasting around 20 minutes — before transitioning into the quieter, deeper stage. The Lullaby Trust notes that, around three months of age, the sleep cycle begins to extend towards 90 minutes.

Why does this matter for night waking?

Because babies cycle through sleep differently to adults, the Lullaby Trust observes that "it's unlikely that your baby will be sleeping deeply at exactly the same time as you." The lighter active-sleep phase at the start of each cycle means infants are more easily roused. The Lullaby Trust also cautions that "encouraging babies to sleep for longer and more deeply than is usual for their stage of development may affect their ability to wake up if something is wrong" — a relevant safety consideration.

What is the link between sleep cycles and SIDS research?

The Lullaby Trust reports that research into sudden infant death syndrome (SIDS) has found that some infants who died from SIDS had "fewer spontaneous arousals and immature sleep patterns compared to other babies," suggesting impaired arousal mechanisms. The ability to wake from sleep — termed autonomic arousal — is an important protective response in infancy.