Is it reasonable to play an acoustic guitar and sing in an apartment in the middle of the day?

It depends entirely on how much sound travels in your building and individual tolerances. Some people will be able to block it out and ignore it but some will get incredibly agitated.  Some flats you won't hear much at all, if anything, others you can hear everything. I live in a flat that's the latter and i play guitar. I just be respectful, dont play for hours on end and play during the day. Ultimately i think the best thing to do is...talk to your neighbours about it. I always let my neighbours know i play and if it ever causes a problem they can come and talk to me and we can sort it out. In my experience it is Much much MUCH better to ask FIRST. Things are SO much more likely to piss people off if you just do stuff and wait for them to tell you it's annoying. If they know they can talk to you, there is a solution ready to go and no need for them to wait till theyre annoyed which will make that interaction harder