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The most important sleep stage is Stage 3, Non-REM or, Delta (Slow Wave) Sleep, it takes up 25% of our total sleep cycle, and it’s known as the ‘deepest’ period of sleep. It’s in Stage 3 that sleep is at its most restorative, helping our bodies heal themselves and our minds rest.

The reason all these wonderful, positive effects take place in Stage 3 is that it is at this point in the sleeping cycle that our metabolism is at its slowest. Our heart rate also decreases as our body really relaxes and sinks into unconsciousness.

At the same time, we don’t take in as much oxygen as we usually do during the day or at any other period of the night. This is one reason why this is the most important sleep stage.

The positive effects of this ‘slowing down effect’ is that our body has time to concentrate on repairing the cells that have been damaged during the day. At the same time, it can also concentrate on powering up our immune system. But it’s not only physical benefits that occur; mentally we are pepped up too.

Our brain goes into ‘fix-it’ mode and files away the day’s experiences to our Episodic memory. It also boosts our personal encyclopedia, i.e. factual knowledge in the part of our brain known as Semantic memory.

And there’s more. The deeper sleep we get the more energetic and happier we feel. Easy to see why Stage 3 is regarded as the most important sleep stage by most of the medical professionals.

Stage 3 sleep then obviously has huge pluses for adults – in fact, it would be difficult to live a ‘normal’ life without it – but it’s for children that it really comes into its own. The reason for this is – yes, you’ve got it – this is when children’s bones and muscles grow. They need it to get taller, fill out, and basically become an adult.

But, not only does Stage 3 sleep restore us, it can also help fend off nasty and potentially fatal diseases such as high blood pressure, diabetes, depression, cardiovascular disease and obesity. There is really no doubt then, about what sleep cycle is most important for us as a species.

Meanwhile, because it’s so deep, Stage 3 is also the reason why some of us manage to sleep through a nearby firework display or even a thunder and lightning storm. It’s also when we’re hardest to wake up. And, if we do get disrupted during Stage 3 sleep, we’re far more likely to feel grumpy and very disoriented – at least for the first five minutes or so. Having said that, some scientists say it can take up to30 minutes to recover from disrupted Stage 3 sleep.

When scientists study sleepwalking and other nocturnal disorders related to sleep, it’s usually Stage 3 in the Sleep Cycle they want to focus on. Not only is it when sleepwalking is most likely to occur, but it is also the stage when kids tend to wet the bed and everyone – juveniles and adults alike – suffer nightmares. Having said that, it’s also the most refreshing period of sleep so if you wake up and jump out of bed feeling terrific, it’s because you’ve probably had more than your fair share of Stage 3 sleep.

It’s when you look at the different periods of Stage 3 sleep during the night that things get more complicated. Although Stage 3 is absolutely the best phase of the sleep cycle for humans, it’s the first two rounds of it during the night that are the most beneficial.

Scientists say we have around four to six periods of Stage 3 sleep a night. The latter periods of Stage 3 are still helpful for our bodies, but they’re just not as effective as the first two episodes. And that’s because, lasting around 25 minutes each, these are the longest two periods of deep sleep. It’s downhill from here in terms of how much Stage 3 sleep you’ll get for the rest of the night.

The amount of Stage 3 sleep we get also decreases with age. Stage 4 REM is also reduced. Both are necessary for helping us deal with memories.

The sleep cycle consists of four different stages. This can be broken down further into REM sleep and NREM (Non-Rapid Eye Movement). The former only occurs at the end of our sleeping cycle (Stage 4) while the other three consist of NREM in different intensities – the third being the most important sleep stage.

Each sleep cycle contains a distinct function. Stage 1 is when we’re just dropping off to sleep. At Stage 2 we are asleep. In Stage 3 we are very deeply in sleep and finally, in Stage 4 our bodies are alert again and we are dreaming.

Each sleep cycle lasts anything from 90 minutes up to 110 minutes and we’d hope for a handful during a night’s sleep.

Most of us are familiar with the light sleeping stage when we’re in bed reading and start to feel nicely drowsy. We put the book down, the lights out and lie there dropping off. It’s also the stage where we’re sitting on the train and nod off, having to jerk ourselves awake. We could also be woken easily at this stage, perhaps not even realising we’ve been asleep.

It lasts for anything from one minute up to seven minutes so only constitutes a small proportion of our sleep cycle (around 20%). During it, though, the heart beat slows, and both our body temperature and blood pressure begin to drop. Our brain waves also slow down and we enter the ‘theta’ stage.

Still referred to as light sleep, Stage 2 in the cycle is an exaggerated version of Stage 1. All of the body states that occur in the first phase deepen here i.e our blood pressure falls further; our breathing becomes even slower etc. It gets harder to wake us, although a slight movement and noise such as the book we were reading falling off the bed could do it.

This is a pretty long stage of sleep –around 50% of the entire sleep cycle.

This is the deep sleep stage and begins around 45 minutes after the onset of Stage 1. When asked what sleep cycle is most important, scientists always point to this one. Now we’re at the healing stage of sleep. This is when the body goes into full repair mode and shouldn’t be disturbed. Scientific recordings clearly show the brain produces long slow waves at this point (known as Delta Waves).

Stage 3 is also the stage we emerge from NREM sleep into REM.

Around 90 minutes after dropping off, REM sleep occurs. Most of us will be familiar (either in real life or on TV) of someone sleeping during REM where you can see their eyes moving underneath their eyelids. Perhaps they’re also talking to themselves at the time or seeming distressed.

Although our eyes are moving, our body doesn’t. Unlike Stage 3 sleep, which decreases as the cycles continue during the night, the periods of REM sleep we get actually increase as the night goes on.

Scientists haven’t really worked out yet what the point of REM sleep is, but it’s believed to relate to problem-solving in the brain and even picking up new skills. Scientists believe the dreams we have during this period help turn short-term memories into longer term data.The thalamus is active in this stage of sleep – the part of our brain responsible for sending our cerebral cortex sounds and images. The latter is the ‘memory processor.’ The amygdala is also busy during REM sleep, helping us process emotions.

Arecent study of more than 4000 middle aged to elderly men showed that a reduction in REM sleep increased mortality. Meanwhile, at the other end of the age scale, babies can spend half of their sleep cycle in REM sleep. This compares with 20% for adults.

Experts say we really need seven to eight hours to benefit fully from sleep. That should allow us to have four to six sleep cycles in a night, making sure we get enough Stage 3 shut-eye - the most important sleep stage - for the body’s ‘repair’ work to get done.

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