Light sleep is the term for sleep that falls into the categories stage 1 and stage 2. It’s a Non-REM form of sleep and differs from heavy (deep) sleep of stage 3. Stage 1 makes up a small fraction of total sleep time (about 3%) and is mainly a transition between waking and Stage 2. Almost all light sleep is stage 2 sleep. Adults spend half (or more) of their nights in light sleep.
Some bodily phenomena are more like waking in light sleep than they are in deep or REM sleep. The person moves more in this stage. If someone is talking while asleep, they are most likely in light sleep. Heart rate and respiratory rate are lower than in waking and the body regulates temperature as it does during waking.
You dream during light sleep, but if you awake and recollect the dreams, they probably will not form any coherent narrative. Light sleep dreams are fragments of images and other brain epiphenomenon (memories, emotion). These are not dreams that form stories, the way that can happen in REM sleep.
Stage 1 is characterized by slow rolling eye movements. On an EEG, stage 2 is characterized by spindles with a frequency of 12-16 Hz that occur in short bursts followed by K complexes. It is also called fast-wave sleep.
Light sleep, or shallow sleep, is called light sleep because it is easier to wake one up during this period. External stimuli such as noise, temperature, touch, and movement can wake us up. But they wake us up more readily, with less effort, when we are in light sleep than in heavy sleep. When people are awakened from Stage 2 sleep, they often deny they were sleeping or claim they were already awake. Sleep inertia is less severe when awakening from light sleep than from REM or deep sleep.
Babies spend hardly any time in light sleep. Sleep spindles that characterize Stage 2 sleep first show in EEGs when a baby is 6-8 weeks old. As the child grows, more of his or her night is spent in light sleep. By mid-teens the individual is spend 3.5-5 hours per night in light sleep, and has established the sleep pattern that will persist into adulthood. Middle-aged people often experience a reduction in deep sleep; the extra time may be spent in light sleep or the total sleep time shortened. Many elderly people have fragmented sleep, dipping in and out of sleep. The sleep they are dipping in and out of is most often light sleep.
Light sleep is perceived subjectively as less refreshing than deep sleep or REM sleep, and this is part of what drives older people to complain about their sleep quality.
Should we look down on light sleep? Is it a lightweight in the list of physiological phenomena? Wasted time? Just filler while your brain is moving between the more productive periods of deep sleep, REM, and waking?
No. Light sleep is useful and an important part of the circadian cycle and sleep architecture. The body rests during light sleep, and cell repair occurs, although perhaps not to the extent that happens in deep sleep. Memory is "backed-up" in light sleep. Neuroscientists strongly suspect that the brain transfers memories from short-term storage to long-term storage during Stage 2, making this form of sleep essential for learning. The "sleep spindles" that show up on EEG readings during light sleep are believed to indicate transmission of facts and memories from one section of the brain to another
"Sleep quality" is not really defined in scientific circles – sometimes this term is shorthand for percentage of time spent in deep sleep or deep plus REM sleep. That implies that light sleep is not quality sleep. It is more correct to say true quality sleep is high efficiency (few or no nighttime awakenings) and a good distribution among the different stages of sleep. Because it is so easy for people to experience light sleep (easier than getting to Stage 3 and REM sleep), the aging brain spends more time in light sleep. It is this imbalance and shortage of deep sleep that people often report as low sleep quality.
Are there such people as "light sleepers"? Yes! Light sleepers and heavy sleepers are descriptions of how likely people are to awaken from external stimuli. Light sleepers may spend more time in light sleep than other people, or more likely, experience the same stages as others, but be more likely to awaken from any of those stages due to stimuli.