If you mean something like a laptop going into sleep mode then there is no way it
can stay connected, nor should it; since it is also left unable to respond to
searches or file requests.

Adding the capability to force a computer to wake up via P2P is not feasible
because that would open a major vulnerability for criminals to exploit. At the very
least it would mean you could never trust a laptop to last overnight without
running down the battery, leaving you without a computer in the morning unless
you were willing to stay there and run on the power cord until it was charged.

If you mean having the node software itself going into a low bandwidth mode
without shutting off the computer then try the legacy versions of BearShare
(4.7-5.1 and possibly a little earlier), which have a Low Power mode which kicks in
after an hour of no activity on the mouse or keyboard. It goes into leaf mode and
reduces the number of host connections to one, which reduces bandwidth and
power consumption to nearly zero but is still ready to respond quickly to download
requests and also keeps your downloads active but slowed down.