Sleep & Fatigue
We need different amounts of sleep at different ages. Babies brains are extremely busy building, reorganising, differentiating, consolidating learning, so they sleep much of the time. A 2 year old still needs 14 hours for brain development and socialization, and a 9 yr old up to 12 hours.
Teen brains are very busy physically pruning and rewiring many emotional and task areas and need 9 hours, which they seldom get thanks to technology, caffeine and a 2 hour shift in melatonin that keeps them awake late evening and asleep when they need to get up for school, leaving them permanently sleep deprived (hence sleeping all weekend). Young adults brains recover well from inconsistent sleep and weekend top-ups, but by 40 we are less flexible and normally need 7-8 hours regularly. Older people have brain changes that can cause sleep to be more fragmented, often worsened by medications and lack of exercise. Luckily they are often able to nap when they feel tired, especially that all important midday nap which we are designed to have.