Final answer:
Stage 1 NREM sleep is a light sleep phase where relaxation is indicated by a shift from alpha to theta brain wave activity, with a reduction in muscle tension and core body temperature.
Step-by-step explanation:
The question refers to the changes in brain wave activity during the transition from wakefulness to the first stage of non-rapid eye movement (NREM) sleep, known as stage 1 sleep. During this initial light phase of sleep, one experiences a transition from alpha waves to theta waves. Initially, alpha waves dominate, indicating a state of relaxation yet wakefulness, characterized by a frequency of 8-12 Hz. As one progresses deeper into stage 1 sleep, there is an increase in theta wave activity, which are lower in frequency (4-7 Hz) and higher in amplitude, signifying the onset of sleep. Moreover, in this stage, there is a marked decrease in both overall muscle tension and core body temperature, making it easy to wake someone from this light stage of sleep. It's noted that individuals with increased alpha wave activity during the deeper stage 3 sleep may not feel as refreshed upon waking.