Most dictionaries define it as a day and a night, a period of 24 hours, but they ought strictly to say that it’s a night and a day, because the Greek roots of the word are nyktos (night) and hemera (day). It’s not infrequently spelled nycthemeron instead and the usual pronunciation, with a th in the middle (as in thin), blurs the join between the two Greek elements.
It was written that way round because in classical times a new day began at sunset, a rule perpetuated in some religions today. Though nychthemeron seems terminally obscure, it is still around, as is its adjective, nychthemeral. Those scientists who study the ways in which the body’s physiological activity varies hour by hour, as in sleep research, make a distinction between nychthemeral and the more common Latin-derived circadian (from circa, about, + dies, a day).
Circadian refers to daily cycles that are driven by an internal body clock, while nychthemeral rhythms are imposed by the external environment. We suffer jet lag because the two cycles get out of step. (“The major zeitgeber [time giver] for our sleep and wake cycle is the nychthemeral alternation of dark and light that occurs on the planet we inhabit.” — William H Moorcroft, Understanding Sleep and Dreaming, 2005.)