Describing the approach of the darkness of night e.g. ‘It’s looking nighter out there’. Importantly, this is distinct from ‘nightier’ which is an adjective that describes a situation in which a nightie is more and more appropriate; this only applies in British English, where as in America the correct adjective to be used would be ‘nightgownier’.
There are many situations that are more suited to wearing a nightie or nightgown. The most obvious is, as the day gets nighter, bedtime and sleep are in our minds and so it feels more nightier (weather permitting of course - for rainy weather you might prefer to wear a waterproof nightie). For this reason, you will never see a shepherd wearing a nightgown, as due to their proximity to counting sheep, it is believed they would be more susceptible to falling asleep were they to attempt their job in a nightgown. Nightgowns endanger lives and the lives of sheep around us.
However, nighties are not limited to bedtime attire and can be worn in different situations - in the theatre, for example, a character wearing a nightie is quite likely to wield a rolling pin and have curlers in their hair, coming out of their house to berate a man beside a bellowing washing line to complain about tardiness or excessive noise; this denotes frustration from the character, and so nighties can be worn to denote frustration.
Similarly, we are reminded of Austin Power, a holy text in this household, in which the 60s spy is accosted by Fembots (that is robots with long hair and, we presume, female genitalia) who are wearing identical pink nighties. In this context the nighties worn are disarming to him, used to hide womanly weapons (in this case breasts with guns for nipples). Therefore, we can think of the nightie as a kind of improvised battle armour for situations in which you might need to employ violence, but hide your intentions. Incidentally, there is a growing marketplace emerging for bras that discreetly house nunchucks, which are often paired with a charming nightie.
The most famous incident of a nightie being worn to hide a threat is in the story of Red Riding Hood. In it the girl, I think she’s called Sandra, is sent out in her distinctive ruby riding hood to walk to her grandmother with supplies after the old biddy decided to live in the woods. En route, a wolf (often represented as big and bad) discovers Sandra’s plans and jogs on ahead, eating the grandmother without leaving any evidence of forced entry or murder, presumably hiding the evidence in his stomach. Notoriously, older women are less nutritious than younger flesh, and so the wolf prepares to eat the girl when she arrives, and wears the grandmother’s nightie as a protective garment to hide his weaponry (fangs, claws, machetette etc.). Due to his attire, he is able to devour the girl - who cares so little about the older generation takes no mind of clear differences in appearance, height, sound and most obviously smell - and we are left with a grizzly scene. Therefore, nighties can also denote a failure in housing policy and the gruesome ramifications of this - either the grandmother should have been housed closer to her tribe, or the woods needed to be better enriched with accessible amenities.
Anyway, the adjective nighter can be used when describing things that are closer to night (8.30 pm until 6.00 am the following morning). You might also use it to compare two different nights - one being more nighter than the other (more moons, darker, higher density of owls and so on).
