Deep Sleep vs REM Sleep and Other Sleep Stages

Non-REM sleep is sometimes called deep sleep, but it is actually three different types of sleep. 

NREM starts when you fall asleep. This is NREM stage 1. This sleep phase is called “light sleep” because most people awake from it easily. During this phase, brain activity slows and some people experience muscle twitches or the sensation of falling (which can sometimes jolt them back to wakefulness). 

NREM 2 is the second phase. Here, heart rate and breathing start to slow, and body temperature falls. Brain activity can sometimes peak, perhaps because of memory consolidation, but then it trails off. 

Finally, NREM 3 is what most researchers call “deep sleep.” During this phase, brain waves are slow and long, and body processes slow to a crawl. If someone is in this phase of sleep, it can be hard to wake them up. The body uses this stage for immune system regulation and deep repair but it is also handy for memory consolidation and physical recovery, especially in people who exercise regularly. 

Researchers often speak of “sleep cycles” that contain both NREM and REM sleep. Most people go through the three stages of NREM sleep, followed by a shorter period of REM sleep, where eye movement is at its highest. 

Here’s how the various stages pan out: 

1. Stage 1 NREM sleep occurs when the eyes are closed and you are losing regular consciousness, but it is still easy to wake you, usually in the first 5 to 10 minutes. 
2. Stage 2 NREM sleep is deeper than stage 1 and lasts for 10 to 25 minutes, during which your body is getting you ready for deeper sleep. 
3. Stage 3 NREM sleep makes up around 25% of your total sleep time 
4. The REM sleep stage usually occurs around 90 minutes after you fall asleep and lasts for around 10 minutes up to one hour depending on the cycle. 

What Is REM Sleep? 

For researchers, REM sleep is one of the most mysterious stages. During this phase, the muscles in your body go limp and you can’t “act out” in your dreams, even if they feel intense. During this phase, the brain becomes more active, stimulating the parts concerned with activities like learning and memory. Evidence also suggests that the brain goes into maintenance mode at this stage, turning short-term memories into long-term pattern recognition.

If you don’t get enough REM sleep, you can start to experience issues. For example, you might have a weaker immune system or trouble concentrating at work. You will also feel groggier in the mornings and have difficulty in regulating your emotions. 

By contrast, you can also get too much REM sleep. Excessive time in this phase doesn’t necessarily cause any long-term health problems, but it can lead to intense, vivid dreams, and disorientation when you wake up (i.e. not knowing where you are). 

How Sleep Trackers Define And Measure Deep And REM Sleep

Modern sleep tracking devices are getting better and defining when people are in deep and REM sleep. However, for non-technical people, the way they work feels like a mystery. How can they measure sleep? 

Sleep trackers actually work by combining various measurable outputs and then inferring the sleep types occurring in your body. Many use sensors found in standalone devices you probably already have lying around your home. These include: 

• Pulse oximeters that measure the oxygenation of the blood over a specific period of time (usually averaging by the minute throughout the night)
• Temperature sensors to determine if core body temperature is falling (and then relating that to the known effects of various sleep stages)
• Heart rate monitors like optical sensors that measure pulse and heart rate variability 
• Sound sensors to detect breathing patterns or snoring
• Accelerometers to measure the type of movement during sleep or periods of stillness associated with deep rest

The next stage is the clever part: feeding these raw sensor inputs into some sort of data analysis procedure or algorithm. These pieces of software can crunch the incoming numbers and then determine the probability of being in a specific sleep phase. 

For example, some algorithms use actigraphy, which is the analysis of the degree of movement you exhibit during sleep. These measurements can determine your level of wakefulness and differentiate light sleep from deep and REM stages. Usually, the less movement there is, the deeper the sleep stage. 

Heart rate variability is another critical consideration of these algorithms. If this metric is steady and slow, it often indicates deep sleep, while REM sleep has specific patterns due to its effects on autonomic nervous system responses. 

Finally, some brands use proprietary algorithms based on in-house sleep testing to compare your sensor data to known patterns that indicate various sleep stages across the population. Brands like Apple Watch, Oura, and Fitbit all use this approach to make their estimations more robust. 

The last phase is sleep stage estimation. Trackers must use their algorithms to come to a final conclusion about the sleep stage you must have been in. 

Deep sleep is usually characterized by a stable heart rate, slower breathing and less movement. By contrast, REM sleep has increased HRV, a bit more movement because of eye twitches, and less regular breathing. 

Are Commercial Sleep Trackers Accurate?

Commercial sleep trackers have come a long way in recent years. However, they are less accurate than more direct techniques, like polysomnography used in labs (EEG-based sleep studies), since these measure brain waves directly. 

Current trackers always use “proxies” to determine sleep depth. As such, their measurements are indirect, so you have to rely on the algorithms they use. These can sometimes be accurate, but they can vary from one manufacturer to another. 

Furthermore, you will sometimes find that trackers tell you that you have had a poor night’s sleep when you feel fresh and ready for the day. Things like irregular sleep patterns or improper wrist placement can affect sensors and prevent them from taking accurate readings. 

Sleep Tracker Stage Outputs

Sleep trackers will usually provide you with a sleep stage breakdown in the mornings. For example, they might say that you have enjoyed 20% deep sleep, 55% light sleep, and 25% REM sleep. Then, they will benchmark that against your wellness goals, telling you how much energy you’ll have for the day. These sleep scores can be handy if you want to optimize your activity levels. 

Now You Know The Difference Between Deep Sleep And REM

Reading this, you should know the difference between deep sleep and REM. Researchers believe they have complementary functions during the sleep cycle, with the former being more about deep repair, and the latter about memory consolidation and processing. Sleep trackers can help you keep tabs on how much time you are spending in these sleep phases so you can determine if you are getting the rest you need.