I am found in the day, but more often at night. Whenever you fall, I may be deep or light. What am I?

The answer to the riddle is sleep. This essential human experience is indeed found in the day, as many people enjoy a refreshing nap, but it is most commonly associated with the quiet hours of the night. The phrase "whenever you fall" cleverly refers to the common idiom "falling asleep," describing the transition from wakefulness into this necessary state. Once you have fallen into it, your experience can vary, as sleep itself is not a uniform state but rather a complex cycle of different stages, which can be either deep or light.

Sleep is a fascinating and vital process, far more intricate than simple rest. During a typical night, your body cycles through several stages, broadly categorized as non-REM (NREM) and REM (Rapid Eye Movement) sleep. NREM sleep includes stages of progressively deeper rest, from light sleep where you are easily awakened, to deep sleep, also known as slow-wave sleep. Deep sleep is particularly crucial for physical restoration, tissue repair, and the release of growth hormones. REM sleep, often occurring later in the night, is characterized by increased brain activity, vivid dreaming, and is important for emotional health and learning.

These cycles, each lasting approximately 90 to 110 minutes, repeat multiple times throughout the night, ensuring that both your brain and body receive the comprehensive restoration they need. Adequate and quality sleep is fundamental for maintaining overall physical and mental health, consolidating memories, and regulating various bodily functions, making it as important to our well-being as food and water.