Sleep is essential to our health, both mental and physical. Doctors have been clear on this point for years. But knowing that you need to get good sleep and understanding what, exactly, good sleep consists of are two different things. And for all their sleep tracking capabilities, smart watches only shed a little extra light on the topic with their use of terms like “light sleep”, “deep sleep” and “REM sleep”. To understand your sleep data, you have to understand these terms and the stages of sleep that they represent.
The American Sleep Association says that there are five stages of sleep. Stages one through four all follow similar trends while the fifth stage, REM sleep, stands apart. REM is so different, in fact, that stages one through four are known as NREM sleep or non-rapid eye movement sleep according to the University of Michigan. We sleep during REM, but our brains are so active that EEGs resemble those taken while awake even though our muscles are completely relaxed. This is why it’s listed separately from light and deep sleep in most cases. Separating those two, as it turns out, comes down to much finer details.
Light sleep vs deep sleep
Light and deep sleep make up the four NREM stages of sleep. As the American Sleep Association explains, stages one and two are considered light sleep while deep sleep does not start until stage three and includes stage four. During light sleep, it is much easier to wake someone. When woken, they can quickly become alert. When in deep sleep, however, sleepers are hard to wake and are often groggy or sluggish when woken. It’s a distinct difference between the two kinds of sleep, but it isn’t the biggest difference.
It turns out that the biggest difference between light sleep and deep sleep comes down to brain waves. Our brain waves slow down a little during light sleep and then slow down considerably during deep sleep. And it is only during periods of deep sleep that our brains emit delta waves, which are high amplitude waves picked up by an EEG. Researchers are still looking into the specific uses of delta waves, as evidenced by the many reports on Science Direct, but they do know that the waves are related to reaction time and processing speed. The American Sleep Academy also explains that when people finally rest after a period of sleep deprivation, their brains seek a rebound in delta waves, indicating that their use is essential to recuperating from sleep loss. And it is these waves that make up the biggest difference between light and deep sleep.
