This is what you need to do:
sudo nano /etc/modprobe.d/8192cu.conf
Add the following:
# Disable power management

options 8192cu rtw_power_mgnt=0

I keep logged into my Raspberry Pi from my Mac. The Pi is connected to my network using a USB WiFi adapter. But when I leave the login idle for a while, I find it to be (a) frozen and (b) I cannot login via a new ssh session – in fact, I can’t even ping the Pi anymore.

I found the solution for problem (b) above in a StackOverflow post.