After periods of inactivity, the wifi connection needs to be reset

If I allow my iMac to sleep .... upon return I must renew the wifi connection (turn it off then turn it on) This happens overnight, of course, but periodically during the day if I leave my computer for an hour or so. Safari can function and the mail app can't get my mail. The fix is always the same, to renew my wifi connection.

Not a disaster but sure is a pitn Would love some advice.

iMac 5k retina display

OS X Yosemite 10. 10. 2

32 BG memory

Graphics AMD Radeon R9 M295X 4096 MB

Thanks!