Final answer:
Stage 3 NREM sleep, also known as deep sleep, is characterized by delta waves and is when it's hardest to wake a person. Stage 2 NREM sleep is associated with sleep spindles and K-complexes.
Step-by-step explanation:
The stage of sleep in which waves are present in young children and during deep sleep is Stage 3 NREM sleep. This stage is characterized by low frequency, high amplitude delta waves. It is in this stage that an individual's heart rate and respiration slow dramatically, making it much more difficult to awaken them. Sleep spindles and K-complexes, on the other hand, are most often associated with Stage 2 NREM sleep, which is a period of deeper relaxation than Stage 1 and is marked by bursts of brain activity that may be important for learning and memory.