On the whole, we are meant to look after ourselves, it is certain
Each has to eat for himself, digest for himself, and in general
Care for his own dear life, and see to his own preservation;
Nature's intentions, in most things uncertain, in this are decisive.
He seems to be saying that if you don't look after yourself you are going against what is natural.
Sometimes when we are stressed and feel we need more strength, more stamina, more wisdom we need to start off with taking care how we eat, drink and sleep.