Hi Robert / PollieXmas
My Open Sprinkler has been working 100% since 2013 without issues.
I did not experience any issues where the WiFi went to sleep.
My issue only appeared after my Raspberry Pi were unable to connect to my access point for an extended time, 07:00 to about 17:30.
I had to reboot the RPi to get it working again, since then my RPi has been online without any issue.
So it seems that if you lose connectivity to you AP for a extended period, the RPi does not re-connect to the AP once the AP comes back on line.