Know when to call it a night. Know when the extra 20 minutes on the project will take more energy than the past 3 hours had combined. You can realise this by listening to the small tug of rest at a point along the way. Healthy progression comes from knowing when to rest and when to trod on, when the stakes a lesser, allow for the natural movements of energy to move you from the active state of work to the active state of rest. Even children need rest from all that playful joy. Tread accordingly.
Meditate.