We spend roughly a third of our lives sleeping, and even though it looks like nothing is going on, there's a lot happening when we slumber. Studies indicate that when we feel sleepy, a chemical called adenosine is starting to accumulate. Neurotransmitters like serotonin signal the brain that it's time to shut consciousness down for repairs. Then, as we become drowsy and fall asleep, we enter the first of five stages.
In stage 1, we're halfway between sleeping and waking, and things could go either way depending on the circumstances. Given an appropriate setting, we start drifting into stage 2, which is where we typically spend 50 percent of our sleep time. During stage 2, eye movement stops and brain waves slow down. Delta waves, which are super-slow brainwaves, start up in stage 3, and by stage 4 all we've got are those slow, slow delta waves washing through our sleeping minds. Stages 3 and 4 are known as deep sleep, and that's when we're so far under it's hard to wake us up. After that, it's on to the REM stage, during which our eyes start twitching around, breathing gets shallower and our muscles are paralyzed. This is when we have most of our dreams. REM lasts for about 25 percent of our sleep time. Then it's back to stage 1, and it all cycles around again. Each cycle lasts around 90 minutes [source: ASA]. That's why it's possible to have sleep terrors more than once in a given night. Each stage appears to have a different physiological function, which could be why we have more than one of them.
Deep sleep (stages 3 and 4) is when our tissues repair themselves and when our body releases hormones, such as the growth hormones necessary for childhood development. The REM stage energizes our brains and enhances our performance of waking life activities.
We mentioned that sleep terrors typically show up during the transition from stage 3 to stage 4, right in the middle of the deep-sleep portion of our night when the delta waves take over and some important hormonal and regenerative activity takes place. This makes them distinctly different from nightmares, which occur during REM sleep. That's also why we rarely remember night terrors, whereas nightmares can remain vividly imprinted on our memories. It's not clear whether those hormonal and regenerative activities have anything to do with sleep terrors.
The literature isn't clear about what exactly happens in the brain during a sleep terror, other than to remark that they seem to be triggered by inexplicable snags in that sleep-stage transition. Those snags, in turn, appear to have something to do with the development of the central nervous system. Studies indicate that the CNS doesn't fully develop until we're 25, which could help explain why some young adults also suffer from night terrors [source: NPR].