That strength, that life force —prana— needs to be managed, less it become vata, undirected or scattered energy. The best kind of management is proactive and conservative.
If we habitually drive a vehicle with the fuel tank on or near empty, the possibility of getting stuck along the way is ever present. However, if we make a habit of refilling the tank every time it approaches the halfway mark, that possibility decreases exponentially.
That is the meaning of the injunction to avoid expending more than 50% of our capacity at any given time. There may be some exceptional moments and situations that require a greater effort, but the rule must be conservation and restoration.
Besides regular sleep, we must withdraw a little from time to time, at least for a few hours, on a regular basis. At times, we must withdraw for a few days, and still others for extended retreat periods. Otherwise, we cannot replenish prana adequately, and inevitably suffer sequelae of impaired function and structural decay.
Each individual must come to know what that halfway mark is, and not transgress it often. But until we come to experience exhaustion with mindfulness, we cannot begin to prevent it.
— Tashi Nyima