Falling asleep when you sit
Moggallāna nodded off while meditating, and the Buddha gave him a list: switch away from the meditation object you were using when drowsiness set in; think over the teaching; recite it aloud; tug your earlobes and rub your arms and legs; wash your eyes and gaze at the stars; attend to the idea of light; walk. If none of that works, lie down with mindfulness, intending to rise, and on waking, get straight up instead of lingering in sleep. It was given to a monk in meditation, and it is practical rather than reproachful. If you are simply short of sleep, rest is ordinary sense rather than this sutta's teaching; if the tiredness is constant, it is worth a doctor's view.
What the suttas say
“But when your harp’s strings were tuned neither too tight nor too slack, but fixed at an even tension, was it resonant and playable?” “Yes, sir.” In the same way, Soṇa, when energy is too forceful it leads to restlessness. When energy is too slack it leads to laziness.
The classic instruction on balancing effort.
AN 6.55:6.1–7.1 · With Soṇa
“Yadā pana te, soṇa, vīṇāya tantiyo na accāyatā honti nātisithilā same guṇe patiṭṭhitā, api nu te vīṇā tasmiṁ samaye saravatī vā hoti kammaññā vā”ti? “Evaṁ, bhante”. Evamevaṁ kho, soṇa, accāraddhavīriyaṁ uddhaccāya saṁvattati, atisithilavīriyaṁ kosajjāya saṁvattati.
“Are you nodding off, Moggallāna? Are you nodding off?” “Yes, sir.” So, Moggallāna, don’t focus on or cultivate the perception that you were meditating on when you fell drowsy. It’s possible that you’ll give up drowsiness in this way. But what if that doesn’t work? Then think about and consider the teaching as you’ve learned and memorized it, examining it with your mind. It’s possible that you’ll give up drowsiness in this way. But what if that doesn’t work? Then rehearse in detail the teaching as you’ve learned and memorized it. It’s possible that you’ll give up drowsiness in this way. But what if that doesn’t work? Then pinch your ears and rub your limbs. It’s possible that you’ll give up drowsiness in this way. But what if that doesn’t work? Then get up from your seat, flush your eyes with water, look around in every direction, and look up at the stars and constellations. It’s possible that you’ll give up drowsiness in this way. But what if that doesn’t work? Then apply your mind to the perception of light, focusing on the perception of day: as by day, so by night; as by night, so by day. And so, with an open and unenveloped heart, develop a mind that’s full of radiance. It’s possible that you’ll give up drowsiness in this way. But what if that doesn’t work? Then walk mindfully, focusing on the perception of before and behind, your faculties directed inwards and your mind not scattered outside. It’s possible that you’ll give up drowsiness in this way. But what if that doesn’t work? Then lie down in the lion’s posture—on the right side, placing one foot on top of the other—mindful and aware, and focused on the time of getting up. When you wake, you should get up quickly, thinking: ‘I will not live indulging in the pleasures of sleeping, lying down, and drowsing.’ That’s how you should train.
The Buddha's graded remedies for drowsiness, given to Moggallāna — change the object, reflect on the teaching, rub the limbs, look at the stars, walk, and finally lie down.
Where this stops: Given to Moggallāna, a monk nodding off in meditation. The last remedy is to lie down with mindfulness, meaning to rise, and to get straight up on waking instead of lingering in sleep; it is not an instruction to rest. For someone simply short of sleep, rest is ordinary sense, not this sutta's teaching.
AN 7.61:2.1–9.4 · Nodding Off
“Pacalāyasi no tvaṁ, moggallāna, pacalāyasi no tvaṁ, moggallānā”ti? “Evaṁ, bhante”. Tasmātiha, moggallāna, yathāsaññissa te viharato taṁ middhaṁ okkamati, taṁ saññaṁ mā manasākāsi, taṁ saññaṁ mā bahulamakāsi. Ṭhānaṁ kho panetaṁ, moggallāna, vijjati yaṁ te evaṁ viharato taṁ middhaṁ pahīyetha. No ce te evaṁ viharato taṁ middhaṁ pahīyetha, tato tvaṁ, moggallāna, yathāsutaṁ yathāpariyattaṁ dhammaṁ cetasā anuvitakkeyyāsi anuvicāreyyāsi, manasā anupekkheyyāsi. Ṭhānaṁ kho panetaṁ vijjati yaṁ te evaṁ viharato taṁ middhaṁ pahīyetha. No ce te evaṁ viharato taṁ middhaṁ pahīyetha, tato tvaṁ, moggallāna, yathāsutaṁ yathāpariyattaṁ dhammaṁ vitthārena sajjhāyaṁ kareyyāsi. Ṭhānaṁ kho panetaṁ vijjati yaṁ te evaṁ viharato taṁ middhaṁ pahīyetha. No ce te evaṁ viharato taṁ middhaṁ pahīyetha, tato tvaṁ, moggallāna, ubho kaṇṇasotāni āviñcheyyāsi, pāṇinā gattāni anumajjeyyāsi. Ṭhānaṁ kho panetaṁ vijjati yaṁ te evaṁ viharato taṁ middhaṁ pahīyetha. No ce te evaṁ viharato taṁ middhaṁ pahīyetha, tato tvaṁ, moggallāna, uṭṭhāyāsanā udakena akkhīni anumajjitvā disā anuvilokeyyāsi, nakkhattāni tārakarūpāni ullokeyyāsi. Ṭhānaṁ kho panetaṁ vijjati yaṁ te evaṁ viharato taṁ middhaṁ pahīyetha. No ce te evaṁ viharato taṁ middhaṁ pahīyetha, tato tvaṁ, moggallāna, ālokasaññaṁ manasi kareyyāsi, divāsaññaṁ adhiṭṭhaheyyāsi— yathā divā tathā rattiṁ yathā rattiṁ tathā divā. Iti vivaṭena cetasā apariyonaddhena sappabhāsaṁ cittaṁ bhāveyyāsi. Ṭhānaṁ kho panetaṁ vijjati yaṁ te evaṁ viharato taṁ middhaṁ pahīyetha. No ce te evaṁ viharato taṁ middhaṁ pahīyetha, tato tvaṁ, moggallāna, pacchāpuresaññī caṅkamaṁ adhiṭṭhaheyyāsi antogatehi indriyehi abahigatena mānasena. Ṭhānaṁ kho panetaṁ vijjati yaṁ te evaṁ viharato taṁ middhaṁ pahīyetha. No ce te evaṁ viharato taṁ middhaṁ pahīyetha, tato tvaṁ, moggallāna, dakkhiṇena passena sīhaseyyaṁ kappeyyāsi pāde pādaṁ accādhāya sato sampajāno uṭṭhānasaññaṁ manasi karitvā. Paṭibuddhena ca te, moggallāna, khippaññeva paccuṭṭhātabbaṁ: ‘na seyyasukhaṁ na passasukhaṁ na middhasukhaṁ anuyutto viharissāmī’ti. Evañhi te, moggallāna, sikkhitabbaṁ.
Still, thinking and considering for too long would tire my body. And when the body is tired, the mind is stressed. And when the mind is stressed, it’s far from immersion.
The Buddha on overwork before awakening — even wholesome thinking, kept up too long, exhausts the body and then the mind.
Where this stops: Said of wholesome thoughts pursued too long; it is about pacing, not a warning against reflection.
MN 19:8.8–8.10 · Two Kinds of Thought
Api ca kho me aticiraṁ anuvitakkayato anuvicārayato kāyo kilameyya. Kāye kilante cittaṁ ūhaññeyya. Ūhate citte ārā cittaṁ samādhimhāti.
It’s when a mendicant—gone to a wilderness, or to the root of a tree, or to an empty hut—sits down cross-legged, sets their body straight, and brings mindfulness to the present. Just mindful, they breathe in. Mindful, they breathe out. Breathing in heavily they know: ‘I’m breathing in heavily.’ Breathing out heavily they know: ‘I’m breathing out heavily.’
The first instruction in the Satipaṭṭhāna Sutta, in full and without elaboration.
MN 10:4.2–4.4 · Mindfulness Meditation
Idha, bhikkhave, bhikkhu araññagato vā rukkhamūlagato vā suññāgāragato vā nisīdati, pallaṅkaṁ ābhujitvā, ujuṁ kāyaṁ paṇidhāya, parimukhaṁ satiṁ upaṭṭhapetvā. So satova assasati, satova passasati. Dīghaṁ vā assasanto ‘dīghaṁ assasāmī’ti pajānāti, dīghaṁ vā passasanto ‘dīghaṁ passasāmī’ti pajānāti,
In the same way, when the mind is restless, it’s the wrong time to develop the awakening factors of investigation of principles, energy, and rapture. Why is that? Because it’s hard to settle a restless mind with these things. When the mind is restless, it’s the right time to develop the awakening factors of tranquility, immersion, and equanimity.
Which factors to develop when agitated versus sluggish.
SN 46.53:12.1–13.1 · Fire
Evameva kho, bhikkhave, yasmiṁ samaye uddhataṁ cittaṁ hoti, akālo tasmiṁ samaye dhammavicayasambojjhaṅgassa bhāvanāya, akālo vīriyasambojjhaṅgassa bhāvanāya, akālo pītisambojjhaṅgassa bhāvanāya. Taṁ kissa hetu? Uddhataṁ, bhikkhave, cittaṁ taṁ etehi dhammehi duvūpasamayaṁ hoti. Yasmiñca kho, bhikkhave, samaye uddhataṁ cittaṁ hoti, kālo tasmiṁ samaye passaddhisambojjhaṅgassa bhāvanāya, kālo samādhisambojjhaṅgassa bhāvanāya, kālo upekkhāsambojjhaṅgassa bhāvanāya.