What does Buddhism teach about energy?

Interesting question; ultimately, as energy is a corporeal realm sort of concept(cf existential sorts of concepts), energy is impermanent and illusory. Of course there 'are' various types of energy, but the actual Teachings of The Buddha & most Branches of Buddhism only don't discuss much about electricity & other forms of energy directly, except indirectly in reference to corporeal concepts, examples of which are nicely cited in several Answers to this Question; and many behaviours, such as eating habits & occupation certainly affect energy. But Buddhist Teaching is more at existential concepts rather than a corporeal sciences guide. Appropriate life conduct & interaction etc is carefully & specifically addressed re following The Noble Eightfold Path & discerning The Four Noble Truths, which are central to Buddhism, and for which literature for which is of course helpfully pervasive & extensive; Textual references translated as energy tend to be more re illusoriness considerations

Suggested Reference Source(of several):

A History of the Philosphy of India, 5 Volumes, S. Dasgupta, 1922-1955, includes meticulously researched & detailed specific doctrinal features of several hundred different Branches of Buddhism(s), and much additional material

Bojjanga is a good starting point to understand this

You can start here: https://www.dhammatalks.org/suttas/SN/SN46_8.html

There is a whole body of knowledge and practice behind bojjanga which is usually well explained by a priest who practiced it well; If you are interested in achieving bojjanga rather than mere curiousity you likely will achieve it to an extent where you don't need others to verify it. To this effect Maha Satipattana Sutta practice 7 years to 7 days applies.

Regarding references from a priest in relation to thripitaka for bojjanga: Ambanpola Gnanawijaya Thero (pardon me for any wrong reference here) is an excellent place to start. Though it is in Sinhalese. (I can consider translation if someone is really keen on practice).

Kaya Sutya MN 5 2 . Unable to find English version

http://www.thripitakaya.org/tipitakaya/Index/138?s=2152

A question about "stamina/fatigue" might be partially answered with doctrine about "torpor".

Here are several references about the causes of torpor -- Denourishing of Sloth and Torpor

One of these is ...

Six things are conducive to the abandonment of sloth and torpor:

1. Knowing that overeating is a cause of it;
2. Changing the bodily posture;
3. Thinking of the perception of light;
4. Staying in the open air;
5. Noble friendship;
6. Suitable conversation.

... though I don't know what that's referencing (perhaps it's a paraphrase or summary, not a quote).

Several suttas mention "not creating new feelings (from overeating)"; and that (i.e. "the middle way") -- was sort of the first Buddhist doctrine even before the Buddha mentioned the noble truths:

• In SN 56.11):

  Mendicants, these two extremes should not be cultivated by one who has gone forth.

• And perhaps more especially in the introduction to the Jataka (not that physical stamina, which is what you're asking about, is the explicit aim).

On the subject of "food" you might see also the Puttamaṃsa Sutta (SN 12.63) which mentions not only physical food, but also sensual contact (feeling), volition (craving), and consciousness (name and form) -- I get the impression that these might be exhausting or fatiguing, in the sense that these might take or involve whatever energy you have.

a collection of all sutta references to sleeping http://lucid24.org/sted/sleep/index.html

The two most important passages occur in these two suttas: AN 3.16 shows the daily moment by moment typical routine, with sleep schedule. (about 4 hours a day sleeping is the typical amount for a monk)

AN 7.61 teaches 7 methods to ward off drowsiness, with sleeping in lion posture the last resort, and not to indulge in pleasures (sukha) of sleeping.

A closely related subject, http://lucid24.org/sted/maranassati/index.html marana sati, shows the attitude of the Buddha towards sleep and making maximum usage of one's viriya/energy to accomplish nirvana before death strikes, and this is in a context of rebirth, where most beings would be reborn forgetting the lessons of the immediate previous life.

"There are these eight grounds for the arousal of energy. Which eight?

"There is the case where a monk has some work to do. The thought occurs to him: 'I will have to do this work. But when I am doing this work, it will not be easy to attend to the Buddha's message. Why don't I make an effort beforehand for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized?' So he makes an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. This is the first grounds for the arousal of energy.

"Then there is the case where a monk has done some work. The thought occurs to him: 'I have done some work. While I was doing work, I couldn't attend to the Buddha's message. Why don't I make an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized?' So he makes an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. This is the second grounds for the arousal of energy.

"Then there is the case where a monk has to go on a journey. The thought occurs to him: 'I will have to go on this journey. But when I am going on the journey, it will not be easy to attend to the Buddha's message. Why don't I make an effort beforehand for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized?' So he makes an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. This is the third grounds for the arousal of energy.

"Then there is the case where a monk has gone on a journey. The thought occurs to him: 'I have gone on a journey. While I was going on the journey, I couldn't attend to the Buddha's message. Why don't I make an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized?' So he makes an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. This is the fourth grounds for the arousal of energy.

"Then there is the case where a monk, having gone for alms in a village or town, does not get as much coarse or refined food as he needs to fill himself up. The thought occurs to him: 'I, having gone for alms in a village or town, have not gotten as much coarse or refined food as I need to fill myself up. This body of mine is light & suitable for work. Why don't I make an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized?' So he makes an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. This is the fifth grounds for the arousal of energy.

"Then there is the case where a monk, having gone for alms in a village or town, does get as much coarse or refined food as he needs to fill himself up. The thought occurs to him: 'I, having gone for alms in a village or town, have gotten as much coarse or refined food as I need to fill myself up. This body of mine is light & suitable for work. Why don't I make an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized?'[1] So he makes an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. This is the sixth grounds for the arousal of energy.

"Then there is the case where a monk comes down with a slight illness. The thought occurs to him: 'I have come down with a slight illness. Now, there's the possibility that it could get worse. Why don't I make an effort beforehand for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized?' So he makes an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. This is the seventh grounds for the arousal of energy.

"Then there is the case where a monk has recovered from his illness, not long after his recovery. The thought occurs to him: 'I have recovered from my illness. It's not long after my recovery. Now, there's the possibility that the illness could come back. Why don't I make an effort beforehand for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized?' So he makes an effort for the attaining of the as-yet-unattained, the reaching of the as-yet-unreached, the realization of the as-yet-unrealized. This is the eighth grounds for the arousal of energy.

"These are the eight grounds for the arousal of energy." - an8.95 https://www.dhammatalks.org/suttas/AN/AN8_95.html

"May nothing remain but skin and sinews and bones; may flesh and blood dry up in the body! Not before having achieved what can be achieved by manly strength, manly energy, manly exertion shall my energy subside!"— MN 70

More on overcoming laziness and drowsiness;

"But, monks, when the mind is sluggish, that is the right time to cultivate the enlightenment-factor of investigation-of-states, the enlightenment-factor of energy, the enlightenment-factor of rapture.[2] What is the reason? A sluggish mind is easy to arouse by these factors. - aggi sutta

Some translations

Viriya - Energy (1,2), Effort (3), Persistence (4), Energy (5)

• Nyanasatta Thera as (1), Piyadissa Thera (2), Burmese Pitaka (3), Thanissaro (4) Bhikkhu Bodhi (5)

Abhidhamma expression;

The energy of his searching, investigating and reasoning out thoroughly that same thing with wisdom, is strenuous, unshrinking. This is called energy-awakening-factor. (3)Therein what is energy-awakening-factor? That which is the arousing of mental energy. right effort, energy-awakening-factor, path constituent, included in the path. This is called energy-awakening-factor.

The mental inception of energy which there is on that occasion, the striving and the onward effort, the exertion and endeavour, the zeal and ardour, the vigour and fortitude, the state of unfaltering effort, the state of sustained desire, the state of unflinching endurance, the solid grip of the burden, energy, energy as faculty and as power, right endeavour—this is the energy that there then is.

Arousing energy from other texts;

Things are conducive to the abandonment of sloth and torpor:

• Knowing that overeating is a cause of it;
• Changing the bodily posture;
• Resolving the perception of light;
• Staying in the open air;
• Noble friendship;
• Suitable conversation. -Vsm

These things, too, are helpful in conquering sloth and torpor:

The recollection of Death; Today the effort should be made, Who knows if tomorrow Death will come?— MN 131

Perceiving the suffering in impermanence

In a monk who is accustomed to see the suffering in impermanence and who is frequently engaged in this contemplation, there will be established in him such a keen sense of the danger of laziness, idleness, lassitude, indolence and thoughtlessness, as if he were threatened by a murderer with drawn sword.— AN 7:46

Sympathetic joy

Cultivate the meditation on sympathetic joy! For by cultivating it, listlessness will disappear.— MN 62

Contemplation of the spiritual journey

"I have to tread that path which the Buddhas, the Paccekabuddhas and the Great Disciples have gone; but by an indolent person that path cannot be trodden."— Vism. IV,55

Contemplation of the Master's greatness

"Full application of energy was praised by my Master, and he is unsurpassed in his injunctions and a great help to us. He is honored by practicing his Dhamma, not otherwise."— Ibid.

Contemplation on the greatness of the Heritage

"I have to take possession of the Great Heritage, called the Good Dhamma. But one who is indolent cannot take possession of it."— Ibid.

How does one stimulate the mind at a time when it needs stimulation?

If due to slowness in the application of wisdom or due to non-attainment of the happiness of tranquillity, one's mind is dull, then one should rouse it through reflecting on the eight stirring objects. These eight are: birth, decay, disease and death; the suffering in the worlds of misery; the suffering of the past rooted in the round of existence; the suffering of the future rooted in the round of existence; the suffering of the present rooted in the search for food. — Vism. IV,63

The five threatening dangers

If, monks, a monk perceives these five threatening dangers, it is enough for him to live heedful, zealous, with a heart resolute to achieve the unachieved, to attain the unattained, to realize the unrealized. Which are these five dangers?

• (1) Here, monks, a monk reflects thus: "I am now free from sickness, free from disease, my digestive power functions smoothly, my constitution is not too cool and not too hot, it is balanced and fit for making effort. But a time will come when this body will be in the grip of sickness. And one who is sick cannot easily contemplate upon the Teachings of the Buddha; it is not easy for him, to live in the wilderness or a forest or jungle, or in secluded dwellings. Before this undesirable condition, so unpleasant and disagreeable, approaches me, prior to that, let me muster my energy for achieving the unachieved, for attaining the unattained, for realizing the unrealized, so that, in the possession of that state, I shall live happily even in sickness."
• (2) Reflect thus: "I am now young, a youth, young in age, black-haired, in the prime of youth, in the first phase of life. But a time will come when this body will be in the grip of old age. But one who is overpowered by old age cannot easily contemplate on the Teachings of the Buddha; it is not easy for him to live in the wilderness or a forest or jungle, or in secluded dwellings. Before this undesirable condition, so unpleasant and disagreeable, approaches me, prior to that, let me muster my energy for achieving the unachieved, for attaining the unattained, for realizing the unrealized, so that, in the possession of that state, I shall live happily even in old age."
• (3) And further, monks, a monk reflects thus: "Now there is an abundance of food, good harvests, easily obtainable is a meal of alms, it is easy to live on collected food and offerings. But a time will come when there will be a famine, a bad harvest, difficult to obtain will be a meal of alms, it will be difficult to live on collected food and offerings. And in a famine people migrate to places where food is ample, and there habitations will be thronged and crowded. But in habitations thronged and crowded one cannot easily contemplate upon the Teachings of the Buddha. Before this undesirable condition, so unpleasant and disagreeable, approaches me, prior to that, let me muster my energy for achieving the unachieved, for attaining the unattained, for realizing the unrealized, so that, in the possession of that state, I shall live happily even in a famine."
• (4) And further, monks, a monk reflects thus: "Now people live in concord and amity, in friendly fellowship as mingled milk and water and look at each other with friendly eyes. But there will come a time of danger, of unrest among the jungle tribes when the country people mount their carts and drive away and fear-stricken people move to a place of safety, and there habitations will be thronged and crowded. But in habitations thronged and crowded one cannot easily contemplate upon the Teachings of the Buddha. Before this undesirable condition, so unpleasant and disagreeable, approaches me, prior to that, let me muster my energy for achieving the unachieved, for attaining the unattained, for realizing the unrealized, so that, in the possession of that state, I shall live happily even in time of danger."
• (5) And further, monks, a monk reflects thus: "Now the Congregation of Monks lives in concord and amity, without quarrel, lives happily under one teaching. But a time will come when there will be a split in the Congregation. And when the Congregation is split, one cannot easily contemplate upon the Teachings of the Buddha; it is not easy to live in the wilderness or a forest or jungle, or in secluded dwellings. Before this undesirable condition, so unpleasant and disagreeable, approaches me, prior to that, let me muster my energy for achieving the unachieved, for attaining the unattained, for realizing the unrealized, so that, in the possession of that state, I shall live happily even when the Congregation is split. — AN 5:78

At last a couple more Sutta excerpt;

“So, mendicants, when the senior mendicants left, why did you sleep until the sun came up, snoring? What do you think, mendicants? Have you ever seen or heard of an anointed king who rules his whole life, dear and beloved to the country, while indulging in the pleasures of sleeping, lying, and drowsing as much as he likes?” “No, sir.” “Good, mendicants! I too have never seen or heard of such a thing. What do you think, mendicants? Have you ever seen or heard of an appointed official … a hereditary official … a general … a village chief … or a guild head who runs the guild his whole life, dear and beloved to the guild, while indulging in the pleasures of sleeping, lying, and drowsing as much as he likes?” “No, sir.” “Good, mendicants! I too have never seen or heard of such a thing. What do you think, mendicants? Have you ever seen or heard of an ascetic or brahmin who indulges in the pleasures of sleeping, lying, and drowsing as much as they like? Their sense doors are unguarded, they eat too much, they’re not dedicated to wakefulness, they’re unable to discern skillful qualities, and they don’t pursue the development of the qualities that lead to awakening in the evening and toward dawn. Yet they realize the undefiled freedom of heart and freedom by wisdom in this very life. And they live having realized it with their own insight due to the ending of defilements.” “No, sir.” “Good, mendicants! I too have never seen or heard of such a thing. So you should train like this: ‘We will guard our sense doors, eat in moderation, be dedicated to wakefulness, discern skillful qualities, and pursue the development of the qualities that lead to awakening in the evening and toward dawn.’ That’s how you should train.” - AN6.17

Drowsiness, lethargy, lazy stretching, Discontent, torpor after meals: Because of this, here among beings, The noble path does not appear.”

“Drowsiness, lethargy, lazy stretching, Discontent, torpor after meals: When one dispels this with energy, The noble path is cleared.” -sn1.16