When talking about Ancient Greek (and Roman) beliefs, it's sometimes important to separate the god of something from a personification of that thing. For example, Hades and Persephone are deities who have power over death, while Thanatos is the personification of death itself; Apollo is a god with solar associations (depending on the time period), while Helios literally is the sun.
Hemera and Nyx are mainly personifications. Hemera is just the Ancient Greek word for "day", and Nyx is the Ancient Greek word for "night". These entities were Day and Night, rather than gods who had power over day and night, and are sometimes better translated that way. There's not much mythology about them, and it's unclear if they were really worshipped.
Astraeus and Eos are borderline, sometimes treated as deities in their own right, and sometimes as personifications of nightfall and daybreak. (Homer, for example, refers to Eos in both ways.) There are indications that they were actually given offerings and such, at least in some time periods.
The Hesperides, finally, are mostly associated with their Garden. There was no ancient consensus about what their individual names were, or even how many they were; what's important is that they tend the garden with the magical golden apples, far off in the west. Their name means "Children of Evening" or "Children of the West" and some authors will link them to different aspects of the sunset, evening, or the west, but like most nymphs they're a literary device more than anything else. Some authors may have invented opposite nymphs to represent aspects of morning, but I'm not aware of any that ever became as famous as the Hesperides.