I have heard a thousand times—maybe, not a thousand times, but several times—from my Tibetan teachers that either they themselves or their teachers studied extremely hard. They either did not sleep at all (at least formally such as by taking off their clothes and lying down on a bed) or that they used to do their so-called dpe klog (i.e. reading) by the moonlight or by glow of a burning incense stick. Of course with the moving shadow, they had to keep on moving, and when the morning dawns they would have found themeless on the top of a mountain! What did they do in the absence of moonlight? They would read the texts with the help of a glow of a burning incense stick! We take light for granted but in the time and place in which they grew up, an artificial light during the night was a pure luxury. Butter lamps, too, must have been a luxury. Incenses must have been a bit cheaper. But can one really read by the glow of a burning incense stick? Yes, one can. I, too, tried when I was a young. It was, in fact, very effective, not because of the brightness of the light but because of the realization that the light is short-lived and that vision of each word or sentence is precious. One cannot, thus, afford to be inattentive.
Actually, I am writing this to take note of a written source that reports of memorizing texts by the glow of a burning incense stick. Ko-zhul, mKhas grub ming mdzod (p. 72): dgung mo spos me ’od la brten nas mda’ tshad ma shog brgyad thugs la ’dzin thub cing….