Final answer:
Stage 2 sleep marks the point of deep relaxation characterized by sleep spindles and is often the first stage when someone can be said to be truly asleep, leading into stage 3 or deep slow-wave sleep marked by delta waves.
Step-by-step explanation:
Understanding Sleep Stages
When discussing the stages of sleep where a person can be said to be truly asleep, stage 2 sleep is often recognized as the point where deep relaxation sets in, following the transitional stage 1 sleep. During stage 2, the body experiences a significant decrease in muscle tension and core body temperature. Brain wave activity, key in defining sleep stages, shifts from alpha waves of relaxed wakefulness to increased theta waves. Sleep spindles, indicative of this phase, are unique bursts of brain activity potentially linked to learning and memory functions.
Advancing further into the sleep cycle, stage 3, also known as delta or slow-wave sleep, signifies even deeper sleep, characterized by low-frequency, high-amplitude delta waves. During this stage, heart rate and respiration slow significantly, and it’s much harder to awaken someone. Therefore, despite individuals being less aware of their environment during stage 1 sleep, stage 2 sleep is the phase where significant physiological changes suggest a person is definitively asleep.