the short answer
Sleep and recovery are the off-training side of progress: training applies a stress, and the body adapts while it rests, repairs tissue, and regulates hormones. Sleep is the highest-leverage recovery input - most adults need 7-9 hours, and short or broken sleep is linked to worse performance, slower recovery, and higher appetite.
This article is general information for coaches, not medical advice. Persistent insomnia, suspected sleep disorders, signs of a nutrient deficiency, or severe or alarming symptoms after training should be referred to a doctor or registered dietitian.