On the origin of Pali thina-middha-(BHS styana-middha-) and Amg. thinagiddhi-/thinaddhi- (Jaina-Skt. styana-grddhi-/styana-rddhi-) the view of Edgerton still prevails: “they go back to OI. *styana-+rddhi-/rddha- ‘increase of languor’ [=Jaina-Skt. styana-rddhi-]> MI. *thina-iddhi-/iddha-; Pali thinam-iddha- with a ‘hiatus-bridging’ m was misunderstood as thina-+middha-; the artificial word middha- has no different sense from that of thina (middha)-”. This theory contradicts our philological research: 1) since the early stage of the Pali Canon (Gatha), middha- and its derivative middhin- are used as simplex with a different meaning from thina-; 2) Amg. thinagiddhi/thinaddhi- is attested only in enumerations of technical terms and is evidently younger than Pali middha-.
Pa. (=BHS) middha- nt. ‘drowsiness, doze’ is supposed to have developed from ved. mrdhrá- (RV+) nt. ‘fatigue, decrease of activity, etc.’, adj. ‘tired, inactive, negligent, etc.’ as well as p. p. mrddhá- (Br.). For want of sleep, monks and nuns were annoyed by sleepiness and doze especially during jhana- (dhyana-) ‘sitting meditation’, which caused a specialization of the sense of middha-. On the other hand, thina- (<*stina-; cf. Skt. adj. styana- ‘coagulated, stagnant’) was used as a nt. noun ‘stagnation/inactivity of mental function (citta-), depression’ esp. with regard to the meditation. As a hindrance to meditation, thina- and middha- combined into a dvandva-compound thina-middha-, rendered styana-middha- in BHS. In the course of time, the sense of the compound was simplified to ‘doze, sitting or walking sleep’ and the beginning consonant of middha- became obscure. Probably in this period, thina-middha- with its synonyms nidda- and pacalayika- was introduced into Jainism, composing a part of Darsanavaraniya-Karman. Contrary to Edgerton's theory, m in middha- was removed to build Amg. thinaddhi- (Jaina-Skt. styana-rddhi-) or replaced with g (under association with giddhi- ‘greed’) to bring forth thinagiddhi- (styana-grddhi-). Since the original sense of middha- ‘sitting sleep’ or ‘walking sleep’ was allotted to payala- (pacala-) and payala-payala (pacalapacala-), thinaddhi-/thinagiddhi- was reinterpreted as ‘action in sleep, somnambulism, etc.’.