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.