Is your onward flight another long haul? If so, I would tend toward not worrying about sleep on YYZ LHR, I don't find it long enough for a restful sleep and regardless you will be exhausted and sleep well on the second flight.

If you're connecting within Europe, try to sleep on the TATL flight then stay awake until "normal" sleeping time at your destination even though you'll want to hit the sack immediately. A late evening departure out of YYZ may help with this.

YMMV.