3. Sloth and Torpor
Through wrong reflection on a state of boredom and the like, sloth and torpor come to be. Boredom is just dissatisfaction. Lassitude is bodily laziness. Languidity of body is the bending of the body torpidly in getting up and in similar actions. Lethargy after a meal is a dizziness or slight faint which is due to eating a principal meal. It is also called the discomfort which follows such a meal. The mind's sluggishness is the dullness of the mind. An abundance of wrong reflection on boredom and similar states of mind produces sloth and torpor. Therefore the Blessed One said that much wrong reflection on boredom, lassitude, languidity of body, lethargy after a meal, and the mind's sluggishness, is a condition for the production of fresh sloth and torpor and the increase and expansion of sloth and torpor already come into being.
Through right reflection in inceptive energy and similar states of mind is brought about the overthrow of sloth and torpor. Inceptive energy is the effort first set afoot. Exertion is more powerful than the inceptive energy because it leaves indolence behind. And because of its assailing further and further of the destructive condition, progressive endeavor is more powerful than exertion. By the exercise of right reflection intensely on this threefold strenuousness sloth and torpor are cast out. Therefore the Blessed One said that the condition for keeping out new sloth and torpor, and for casting out sloth and torpor that is old, is abundant right reflection on the element of inceptive energy, of exertion and of progressive endeavor.
There are six things which lead to the casting out of sloth and torpor: The seeing of the reason of sloth and torpor in the fact of eating too much or gluttony; the changing of the postures completely; reflection on the perception of light; staying in the open; sympathetic and helpful companionship of the good; and stimulating talk that assists in dispelling sloth an torpor.
There is the following explanation of these six things: The bhikkhu who has eaten gluttonously is assailed by sloth and torpor while doing his recluse duty of meditation in his day or night quarters as by a mighty elephant pressing down on him, but that one who practices moderation in food is not troubled thus with these hindrances. In one who thus sees the characteristic of sloth and torpor in gluttony there is the casting out of sloth and torpor.
Sloth and torpor disappear in him who changes over from the posture which induces sloth and torpor to another; in him who reflects on the brightness or the light of the moon, a lamp or a torch by night, and on the light or brightness of the sun by day; in him who lives in the open; in him who associates with sympathetic and helpful companions, like the Elder Maha Kassapa, who have dispelled sloth and torpor; and by stimulating talk connected with a strict recluse-regimen.
Therefore it is said: Six things lead to the casting out of sloth and torpor. The yogi understands thus: sloth and torpor cast out by these six things are stopped from arising forever in the future by the attainment of the path of arahantship.
The bhikkhu who has eaten gluttonously after the manner of the well-known types of Brahmanical gormandizers mentioned in ancient Indian books. There are five kinds of these greedy eaters: (1) He who eats until he has to be raised up by the hand from his seat. (2) He who lies rolling just where he has eaten and eats as long as he likes. (3) He who eats until he slips off his waist cloth. (4) He who fills himself with food in such a way that it seems as if a crow could peck at the food in him. (5) He who having filled his belly full and vomitted eats more food again, or he who eats until he vomits.
On the light or brightness of the sun by day: The meaning should be understood thus: Sloth and torpor vanish in him, too, who at night is reflecting on the image of the perception of the brightness of the sun he got by day.
Here it may be helpful to state the eight ways of dealing with torpor taught by the Master to the Elder Maha Moggallana: (1) One should neglect to mind the thought which says that drowsiness is descending on one, or (2) one should reflect on the Dhamma, or (3) repeat or recite the Dhamma, or (4) pull both earlobes and rub or massage the limbs with the hands, or (5) getting up from the sitting position, apply water on and rub the eyes, and look into the distance, at the constellations in the starry sky, or (6) reflect on the thought of light, or (7) fix the thought on the ambulatory, aware of the ends of it with the controlling faculties of sense turned inwards and the mind kept in, or (8) sleep conscious of the time of waking and on awaking get up quickly thinking that one will not give oneself to the comforts of lying down, reclining and languor, when all other seven ways fail.
I think it is as simple as you mentioned, i.e. to just imagine a bright light or imagine the items described in the quote.
