#1 for health is some form of chi cultivation method such as posted above by Egon.
Good second place is reserved for pranyama, that is breathing exercises.
Those alone, when practiced every day will bring great improvement. There is more to 8-fold path of course, but the above are very beneficial for health in many ways.
The power of pranayama, like the breath of fire, is that it is very very easy to do. People who are already tired don't want to do a full yoga session, even if is the full solution.
People with lower vital reserves need to be aware of proper energy management and what to do in situations of low energy. In this case, I would recommend the breath of fire and holding stomach 36 to promote digestion. Otherwise, just take a nap.
Once this person has risen above this "life-support" level of energy, then they will usually feel ok enough to do a full yoga routine. This then takes them back to optimal levels of vitality.