Thanks Malcolm; but does this suggest that time spent in "certain dhyanas" will forstall some of the aging process and thus lengthen life of the body somewhat?
Could there be also a purpose, for very advanced practitioners, that those days or weeks of tukdem be used to seek another physical body to transfer to? I have forgotten the name of this practice, from the Six Yogas of Naropa, I recall.
For as long as the life indriya is not separated from the body, it will not decay. The separation of the life indriya is prevented by being in a state of samadhi at the time of death.
Related to this, according to the Abhidharmakośa one can extend one's lifespan (āyurvipāka) through a process of dhyāna and generosity where the yogi transforms retribution-in-joy (bhogavipāka) into an extension of life. See the following quote:
śāstre uktam ——“kathamāyuḥsaṃskārān sthāpayati ? arhan bhikṣuḥ ṛddhimāṃścetovaśitvaṃ prāptaḥ saṅghāya vā pudgalāya vā pātraṃ vā cīvaraṃ vā anyatamānyatamaṃ vā śrāmaṇakaṃ jīvitapariṣkāraṃ vā dattvā tat praṇidhāya prāntakoṭikaṃ caturthaṃ dhyānaṃ samāpadyate|sa tasmāt vyutthāya cittamutpādayati vācaṃ ca bhāṣate ——‘yanme bhogavipākaṃ karma tadāyurvipākaṃ bhavatu ’ iti tasya yad bhogavipākaṃ tadāyurvipākaṃ bhavati| yeṣāṃ punarayamabhiprāyaḥ ——vipākoccheṣa vipacyata iti|
The Mūlaśāstra says: “How does a Bhikṣu stabilize the vital energies? An Arhat in possession of supernormal power (ŗddhimān-prāptābhijñāḥ), in possession of mastery of mind, i.e., one who is asamayavimukta, gives, either to the Sangha or to a person, things useful to life, clothing, pots, etc.: after having given these things, he applies this thought to his life; he then enters into the Fourth or prāntakoṭika Dhyāna; coming out of the absorption, he produces the thought and pronounces the words: 'May this action which should produce a retribution-in-joy [bhogavipāka] be transformed and produce a retribution-in-life [āyurvipāka]!' Then the action (the gift and the absorption) which should produce a retribution-in-joy produces a retribution-in-life.”(1)