Final answer:
Matt is most likely in the REM sleep stage, indicated by his darting eye movements and quivering eyelids, which are typical characteristics of this phase of sleep.
Step-by-step explanation:
Matt is likely experiencing rapid eye movement (REM) sleep, a period of sleep that is characterized by brain waves very similar to those during wakefulness. This phase of sleep is marked by the darting movements of the eyes under closed eyelids, which is what Matt is displaying. Unlike night terror sleep disorder, REM sleep is generally not associated with a sense of panic or the need to scream or escape. It's important to note that rapid eye movement doesn't involve physical activity outside of the eyes as seen in REM sleep behavior disorder (RBD), where muscle paralysis does not occur and individuals may move significantly during sleep.