Sure, you reduce the power, but also will have spurious retransmissions of packets because of lower power, so all connections will be poor.

Also aside from turning it off, a lot of router/AP combos do support WiFi schedules, so that the radios can be disabled on a timer when you're sleeping.

If you don't want the radio rattling around, learn how to do some network cabling. :)

Wasting perfectly good aluminium foil.