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?
I wouldn't worry about it. The reason clients are using Upwork is because it enhances their efficiency - for example, hire a freelance writer, accountant, or programmer because they don't have the capital to keep one on staff full-time.
That said, being able to post a job freely also allows freelancers to bid on it freely, removing the "hiring process" barriers typically found in a traditional business.
Also worth noting is that if the job is for an already employed individual, their schedule may be such that evenings and weekends are their prime hours to work on a side project.
I would submit the proposal as soon as you see a listing that fits. You're competing against other freelancers for the same work, and sometimes the right attitude in a proposal is all it takes to win a client over.
I'd also add that if you're submitting a proposal to a business, they may not view it until the next business day so plan for a day or two at a minimum.