Which two types of waves are primarily observed in deep sleep?

Deep sleep, or slow-wave sleep, is characterized primarily by delta waves. These waves are high-amplitude, low-frequency brain waves, which are typically observed in stages three and four of NREM sleep. Delta waves indicate that the brain is in a state of deep rest, and they play a critical role in restorative sleep functions such as tissue growth and repair.

Theta waves, though present in lighter stages of sleep and during drowsiness, are not primarily associated with deep sleep. They are often observed during the transition from wakefulness to sleep and in lighter stages of sleep. The combination of delta and theta waves can sometimes be seen during transitions between sleep stages, but the predominance of delta waves is key in defining deep sleep.

Alpha and beta waves are associated with wakefulness and alertness, while gamma waves are linked to higher cognitive functions. Therefore, in the context of deep sleep, the presence of delta waves serves as the defining characteristic, supported by the presence of some theta waves in the earlier stages of sleep but not as the primary waves defining deep sleep. This understanding cements the choice of delta and theta waves as the primary waves observed in deep sleep.