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Understanding Your Baby’s Sleep Cycles

Understanding Your Baby’s Sleep Cycles

online baby sleep course for ages 5 months to 12 months

Sleep cycles are something that we see written and spoken about a lot but, ultimately, I think a really good understanding of what a sleep cycle means and how it affects your baby’s sleep is really quite important to know. As well as the dreaded and much awaited 4 month sleep regression – is it a regression or not?

There’s tons of research and scientific literature out there on sleep cycles and I hope that this blog post will give you an overview of what it all means in real terms and how sleep cycles play a role in your baby’s sleep. I will also touch on the 4 month sleep regression and all of the myths surrounding it.

Sleep Cycles

A sleep cycle is a part of yours and your baby’s internal biological clock. It consists of regular patterns of brain activity that happen when we sleep. It’s really interesting that there are many studies that show your baby will spend around 95% of their later weeks in the womb sleeping – those last 4-6 weeks they spend a lot of time asleep and that’s most likely down to the massive vast increase of brain development in that time. This helps to set them up for birth and all the new experiences once they come out of the womb. Not only that, babies are also moving through sleep cycles whilst still in the womb. Studies using fetal heart rate trackers have shown that babies do demonstrate really regular sleep and waking patterns in the womb so they’re already going through the motions before they’re even born!

How long is a sleep cycle and how does it impact their sleep?

For a baby a sleep cycle is somewhere between 35 – 50 minutes on average. Many clients  have said to me “Heidi I need help with daytime sleep, my baby is only doing 38 minute sleep cycles!” Already, before clients come to me the client is already an expert at their baby’s sleep and they know how long their sleep cycles are! When a baby comes to the end of a cycle they can often wake up, rather than go back to sleep. Some parents can feel obsessed by this and know, to the second, when their baby will wake! I have also had clients whose baby has slept brilliantly until four months or five months and then babies suddenly wake after a sleep cycle and they struggle to settle again. 

As an adult, our sleep cycles are 90 minutes long – they double in time from when we are babies to adulthood. When your child becomes a toddler and a school-age child, their sleep cycle will always be lengthening. 

Other factors can affect their sleep cycle and whether they can link them or not.

How a sleep cycle works:

Stage 1: where your baby begins to get drowsy and they fall asleep.

Stage 2: what we like to call your “active sleep” (or REM sleep) – the sort of sleep where your baby may move around a lot in their sleep and they might twitch a lot or they might look erratic.

Stage 3: when the light sleep occurs but your baby is moving less than stage 2, this is a transition through to deeper sleep. In this stage of sleep, they can be easily woken up. 

Stage 4: deep sleep. Your baby is so much less likely to wake at this phase of sleep. Very little will wake your baby during deep sleep – even the doorbell! They will be quite still with slow, rhythmic breathing at this stage.

They come through to light sleep from stage 4 or they will wake up or – if you’re very lucky – they will fall back into another sleep cycle. Half of their sleeping time is spent in stage 2, which means it’s easy to mistake them for not sleeping properly. This is something I always try to impress on new parents because movement doesn’t always mean they’re waking up!

Adults spent 20% of their time in light sleep and 80% in deeper sleep. The difference between adults and babies is down to survival: babies need to be able to safely rouse themselves to tell their parents that they need something.

In those first few weeks and months sleep cycles are not hugely defined and you won’t notice your baby going through the motions of sleep cycles. But when they get towards 2 or 3 months, their sleep cycles become quite obvious to you and to a lot of people this leads to the 4 month sleep regression that everyone talks about.

What is the 4 month sleep regression?

I know that people dread the idea of the 4 month sleep regression and I’m often asked what it is by clients. A baby’s sleep cycle becomes quite clearly defined around 4 months of age, as the brain goes through a developmental leap, so your baby becomes much more aware of going between the sleep cycles.

This means that each time they come into light sleep, at the end of a cycle, they have a survival or a protective mechanism that causes them to stir slightly to check that they’re still safe. Obviously nowadays, we are all – mostly, of course – safe but think back to the caveman days and you can begin to understand this very human part of our development. If a baby was in Mum’s arms or on the breast when they fell asleep or perhaps they had a dummy but it falls out of their mouth during sleep – when they do their ‘check in’ at the end of the sleep cycle they realise that something might be different and this can cause them to feel unsafe so they cry out or won’t resettle until they get that thing that had when they fell asleep.

The 4 month regression is really a PROGRESSION and an important part of their growth and development.

Things can influence how easily your baby goes through sleep cycles and there’s lots you can do to help them stitch their sleep cycles together:

  1. Their age – it’s much easier for them to link their sleep cycles in the first couple of months.
  2. Hormones – how much sleep hormone a baby has in their body. Babies have more melatonin (the sleep hormone) around 6-7pm and so they’re more likely to sleep and stay asleep around this time. As melatonin levels drop towards the second half of the night, their sleep will become lighter.
  3. Comfort – is your baby comfortable or is there something that’s bothering them? Imagine if you were feeling unwell and had a painful mouth ulcer, this would naturally make it more difficult for you to sleep and to move into deeper sleep. It’s no different for a baby: if they’re being bothered by something, such as tummy discomfort, wind or pain from stomach acid and reflux, then when they come into light sleep they are much less likely to be able to go back to sleep or join their sleep cycles very easily.
  4. Tiredness levels – if your baby is exhausted and overtired, their body is producing cortisol and will keep them wide awake and it will take them a little longer to fall into deep sleep. Once they come to the sleep cycle ending they’ll wake up, as it’s harder for them to join sleep cycles. Try to follow their awake cycles really close and don’t put them down too late.

I explore how to follow a baby’s awake windows, sleep signals and much more in my online sleep course, which you can find out about here.


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