In a study of 565 adults in Poland, the time of peak desire and the time of sexual activity varied by chronotype and sex. The link between when participants wanted sex and when it happened was only moderate. Many reported being sexually active in the evening, but the sample was limited to one country and relied on self-reports; it does not establish an ideal time for everyone or show that the body clock alone drives desire.
In a relationship, it may help more to compare your actual rhythms than to guess the “right” hour: when is each of you usually alert, unhurried, and able to focus on being together? Sometimes the answer is not a sex schedule but finding a time when neither person is exhausted. And if your rhythms do not match, that does not mean you are incompatible; it is a reason to look for an arrangement that works for both of you, not to blame yourself or your partner for lacking desire.