I agree that the people who have been through the most do have the best advice (mostly). This is because of experience - it's like going through the 'school of hard knocks' - when you've been through it, you'll learn how to deal with it and not to make the same mistakes again. Although I say 'mostly' as it all comes down to personal character and resilience. One person going through something may get up and it'll make them stronger and wiser, whereas another person may go through the exact experience and think 'woe is me, it's me against the world' and fall lower by turning to things like alcohol/drugs etc. So it depends on the individual and who you're getting the advice from.

A good laugh and long sleep are great for your health - not necessarily 'cures for anything'. Logically both activities helps the brain to release 'happy-chemicals' which overall are great for the immune system, health and well-being. It can also help mask feelings of pain, sadness and other mental health problems. The reason why I say it doesn't 'cure' anything is that - a better term is 'preventative measures' - this is because if someone is already chronically ill then they'd need a little more than sleep and laugher to get better.