Ejaculation during sleep happens without explicit masturbation.
According to Hinduism, masturbation is a sin.
But does Hinduism classify ejaculation during sleep as a sin?
Atonement measures are prescribed only for actions which are considered as sins.
And, for nightfall the Manu Smriti does recommend one such measure.
स्वप्ने सिक्त्वा ब्रह्मचारी द्विजः शुक्रमकामतः ।
स्नात्वाऽर्कमर्चयित्वा त्रिः पुनर्मामित्यृचं जपेत् ॥ १८१ ॥
svapne siktvā brahmacārī dvijaḥ śukramakāmataḥ |
snātvā'rkamarcayitvā triḥ punarmāmityṛcaṃ japet || 181 ||
The twice-born religious student, haying unintentionally dropped his manhood during a dream, should thrice recite the three vedic verses beginning with “punarmām,” after having bathed and worshipped the sun.—(181)
So, it seems to be a sin of a very small magnitude.
Samvarta Smriti however says only bathing is enough to remove that sin.
A Brahmacharin,. who knowingly discharges his seminal fluid, should perform the expiation consequent on the breach of the vow ; and if unwillingly, he should be purified by bathing.
(Verse 28 of the first Chapter)
If you see it from another angle, then nightfall can be a result of a disease and diseases don't affect a sinless person. So, although it's unintentional still it's a sin.
Medhatithi's commentary on the Manu Smriti verse from the linked page is as follows:
When one renounces his vow of continence intentionally, then he has to perform the expiatory rite prescribed for the ‘Avakīrṇī’ (11.120 et. seq.); the present verse lays down what one should do when he does it unintentionally.
No significance attaches to the mention of ‘dream’; the absence of intention is the only necessary condition; and no intention can be present during dreams. Hence this same expiatory rite is to be performed in a case where, even though he may be not asleep, the flow occurs involuntarily, in the same manner as certain other fluids flow out of the body.
The sense of the verse thus is that—‘if one drops his manhood unintentionally, he should perform this expiatory rite that he should recite the three verses, etc., etc.—(181)
So, the first point is that the verse and the injunction is applicable only to students (those who are residing in the Brahmacharya Asrama) and the second point is if it was not a sin for them no expiation would have been necessary.
But expiation has been laid down.
No, Nightfall is not considered as sin in Hinduism.
17:25 Someone of the vow of celibacy should never spill his semen and, when it flowed of its own in sleep, take a bath, control his breath and chant the Gâyatrî
The loss of reproductive elements dissipates a person’s strength. But there is no harm in nocturnal emission. That reproductive element is from food. After nocturnal emission, a man retains enough to succeed. But he must not lose it voluntarily. What remains after nocturnal emission is very refined. The Lahas stored pots of molasses with holes in the bottom. After a year it was found that the molasses had turned into crystals, like sugar candy. Whatever liquid there was had come out through the holes.
It is very harmful for a spiritual aspirant to discharge vital elements. He has to be very careful not to even look at a woman (man for female aspirants). Even if she is a devotee, he must leave that place. Even to see the picture of a woman might cause the discharge of vital elements in a dream, if not in his waking hours.