One of the sons born of the womb of Aditi, mother of the gods, was Vivasvān. Vivasvān is another name for Sūrya, or the Sun-god. Sanjñā was one of the three wives of Vivasvān. She was the daughter of Tvaṣṭā. Unable to bear the energy and power of Vvasvān, Sanjñā created a clone of herself, called Chāyā. Entrusting her with the responsibility of looking after her husband and children, Sanjñā left the home. Sanjñā had three children– two sons called Manu and Yama, and one daughter called Yamunā. In course of time, Chāyā also gave birth to Sāvarṇi, Manu, Śani and Tapatī. Chāyā looked exactly alike Sanjñā, but she showed more affection towards his own children. Of the children of Sanjñā, Yama protested against this discriminatory behaviour and got cursed by Chāyā in turn.

This kind of hostile behaviour cannot be of one’s own mother– thus Yama complained to his father Vivasvān,. Vivasvān could realise the behaviour of Sanjñā, and went to Viśvakarmā, asking him to reduce some of his energy. Viśvakarmā placed Sūrya on a whirling machine and some of his energy got reduced. Vivasvān was doing all this for Sanjñā, but she was then roaming about the Merupradeśa, assuming the form of a mare called vaḍavā. Vivasvān came down to the earth, and he also took the form of a horse, coming face to face with Sanjñā. Out of love and desire for consummation, he put his mouth on Varavā’s mouth, but Sanjñā-Vaḍavā could not recognise him as her husband, so she got disgusted. But already, the vital energy of Vivasvān entered into the nostrils of Vaḍavā. Considering the horse to be another man, Sanjñā threw off that vital energy. From that energy, coming out of her nostrils, emerged the two Aśvinikumāra(s). Since they were born of Aśvinī(the mare) — so they are sons of Aśvini. And they are also the children of Vivasvān in the form of the aśva(horse). Again, they were born out of the sputam coming out of the nose, so they were Dasra, or Dasrau in Dvivacana. Again , they were born out of the tip of the nose, so they are called Nāsatya, or Nāsatyau in Dvivacana–
tataḥ sa bhagavān gatvā bhurlokamamarādhipaḥ
kāmayāmāsa kāmārto mukha eva divākaraḥ.
aśvarūpeṇa mahatā tejasā ca samāvṛtaḥ
sanjñā ca manasā kṣobhamagadbhayavihvalā
nāsāpuṭābhyāmutsṛṣṭaṃ paro’yamiti saṅkayā
tadretasambhūta jātāvaśvināviti niścitam
dasrau skratatvāt sanjātau nāsatyau nāsikāgrataḥ.

In Viṣṇupurāṇa, the legend is almost similar. Only Sanṝjñā here is not the daughter of Tvṣṭā, she is is directly the daughter of Viśvakarmā. Here also, unable to bear the radience of Sūrya, Sanjñā has gone away, leaving Chāyā behind, the the curse of Chāyā comes upon Yama. Both Yama and Sūrya suspected Chāyā, and finally they came to know from Chāyā about Sanjñā’s escape. Now, in his meditative state, Sūrya found Chāyā as a mare, practising tapasyā inside a forest. He then came to that forest, assuming the form of a horse, and consummated with the Aśvinī Sanjñā, and the twin Aśvinikumāra(s) were born—
tato vivasvānākhyāte tayaivāraṇya sansthitām.
samādhidṛṣṭyā dadṛśe tāmaśvāṃ tapasi sthitām
vājurūpadharaḥ so’pi tasyāṃ devāvathāśvinau.

In Viṣṇupurāṇa, Viśvakarmā reduced the energy of his son-in-law Vivasvān after the birth of the Aśvinikumāra(s). In Harivaṃśapurāṇa and Mārkaṇḍeyapurāṇa, the account of the mating between Vivasvān Sūrya and Sanjñā, has been narrated in a similar way, though in these two texts, more importance is given to the nasal consummation between Vivasvān and Sanjñā. Especially, the fear of Sanjñā,considering the advances of ‘another man’, and the account of Sūrya’s vital erection into her nostrils, and the two names of the Aśvinikumāra(s)– Nāsatya and Dasra– these get more importance–
devau tasyāmajāyetām aśvinau bhiṣajāṃ varau
nāsatyaśvaiva dasraśca smṛtau dvāvaśvināviti.
It is to be noted that in the description of Purāṇa(s), somewhere both are called Nāsatya and dsra, and elsewhere, one is called Nāsatya, the other Dasra.

In Prabhāsakṣtra-māhātmya of Skandapurāṇa, the story of the birth of the Aśvinikumāra(s) has been all the more similar to what has already been narrated. But in the Avantīkṣetra-māhātmya of the same Purāṇa, the name of the wife of Vivasvān Sūrya is not Sanjñā, she is Sāvitrī, and Tvaṣṭā, the Vedic name for Viśvakarmā, has been used as the name of his father. Tvaṣṭā gave Sāvitrī to the hand of Sūrya–
purāṇusūryāṃ sāvitrīṃ tvaṣṭā svatanayāṃ dadau.
This Sāvitrī, after the birth of Yama and Yamunā, left them to the hand of her self-projection Chāyā. She was still very much in fear of Sūrya’s energy– saviturbhayavihvalā.

However, Sāvitrī did not even stay in her father’s house, she went on roaming, in the shape of Vaḍavā. In the meantime the conflict between Chāyā and Yama made Vivasvāna doubtful. The angry Sun-god came to his father-in-law, but unable to give any information regarding the whereabouts of Sāvitrī, Tvaṣṭā lamented and blamed Sūrya in turn, saying that — “My daughter went away, without being able to bear your energy. If your wife is really dear to you, you should rather reduce some of your energy–
tava tejaḥparibhraṣṭā bhagnā kvāsi gatavalā
yadu te vallabhā bhāryā tejastvaṃ pariśāmaya.

Sūrya was finally agreed to reduce his energy, and Tvaṣṭā, using Sudarśana cakra as a sharpener, Tvṣṭā made Sūrya lighter and soft. Then, Tvaṣṭā gave him the address of Mahākālavana and asked him to go to Sāvitrī, in the guise of a Varavā. Sūrya came to the place where the river Kṣātā and śiprā meet, and found Sāvitrī roaming about as vaḍvā. Vivasvān Sūrya then took the shape of a horse, and took the smell of vaḍavā̎s nostrils, and as a result, she gave birth to two sons. They were handsome to look at, and they were appointed as the physicians of the gods–
nāsikā-ghrāṇamātreṇa yatra jātau sutāvubhau
darśanīyau sunimnāṅgau bhiṣajau tau divaukasām.
In Mahābhārata, the birth-story of Aśvinikumāra(s) is not discussed in details. But as it is found in Purāṇa(s), and since its suggestion can be found in Ṛgveda itself, so Mahābhārata mentions the Vedic names of the two Aśvinikumāra(s), and recognises the two separate identities of these two gods-
nāsatyaścāpi dasfraśca smṛtau dvāraśvināvapi
mārtaṇḍasyātmajāvetau sanjñānāsāvinirgatau.

In Mahābhārata and Purāṇa(s), the birth-story of the Aśvinikumāra(s) has a Vedic basis to it. In Ṛgveda we find that when the god called Tvaṣṭā was getting his daughter Saraṇyu married, all the universe assembled to watch that wedding. When the mother of Yama, wife of the great Vivasvān, was getting married, she became invisible. Then another woman, like the immortal Saraṇyu was created, and given to Vivasvān. Then Saraṇyu got pregnant with the twin Aśvinikumāra(s), and gave birth to two sons–
tvaṣṭā duhitre vahatuṃ kṛṇotītīdaṃ viśvaṃbhuvanaṃ sameti.
yamasya mātā paryuhyamānā maho jāyā vivasvato nanāsa.
apāgūhannamṛtāṃ martyebhyuḥ kṛtvī sarvaṇāmadadurvivasvate
utāśvināvabharadyatta dāsīdajahādu dvā miyunā saraṇyuḥ.

These two Ṛk mantra(s) are the source of the later Purāṇa-legends. Tvaṣṭā got his daughter Saraṇyu married to Vivasvān, and that Saraṇyu has become Sanjñā in Purāṇa, and sometimes Sāvitrī. Behind their invisibility, there is a Paurāṇika truth. Sanjñā Sāvitrī could not bear the terrible energy of Sūrya, and so comes the notion of her self-concealment. This legend is not a product imagined by the narrators of Purāṇa(s), we get its proof from the ancient text of Nirukta by Yāska. While explaining the above-mentioned Ṛk, Yāska has said, Tvāṣṭrī, Tvaṣṭā̍s daughter, after giving birth to Yama and Yamī, left one like herself (savarṇām anyām) as her representative, and escaped, assuming the shape of a mare. Vivasvān Sūrya also took the shape of a horse and consummated with Saraṇyu, and gave birth to the twin Aśvinikumāra(s). And Vaivasvata Manu, son of Vivasvān was born of the womb of her like-personality Savarṇā–
sā savarṇāmanyā pratinidhāyāśvaṃ rūpaṃ kṛtvā pradudrāva
sa vivasvān ādityaḥ savarṇāyāṃ manuḥ.
In another Vedic text like Vṛhaddevatā, this incident is narrated in a verse-form, and this narration has a speciality. In Avantīkṣetramāhātmya of Skandapurāṇa, the story of the birth of the two Aśvi(s), is narrated in a different way. Here the female initiative is highlighted. According to the description of Vṛhaddevatā, Saraṇyu recognised Vivasvān, and wished to engage in copulation out her her own will. Vivasvān rode upon her, but of of excitement and desire, his semen fell on the ground. Saraṇyu, willing to conceive, smelled that semen, and immediately got pregnant with the Aśvin(s), who were born as Nāsatya and Dasra–