You can submit proposals at any time, 1 am, (EST) 4 am, on Sat or Sun and even on holidays such as Christmas, Easter etc. Clients around the world post jobs at all times, day and night, weekends, and also on holidays. and received proposals at all times.
Hi Eda. I don't think it's unprofessional. It's not like a phone call, which is hard to ignore. Clients can check their Upwork messages and emails at a time of their choosing. Some clients do work at the weekend, so you may be missing an opportunity if you wait.
Besides, Upwork is international. Would you also limit yourself to making proposals during the client's office hours, even if that happens to be your night time?