Tanks shouldn't be in the clump that is getting slept or you're bad.

You also can break sleep using purify, you can also use purify on a slept target. You can healer lb immediately after the first cometeor, since it breaks sleep to negate at least 1 cometeor.

Outside factors can also break sleep. A pld from the other team that scorned, a shadowflare on the ground people are standing in, the node AE.

Additionally, you can also have good melee or tanks or a caster with a well timed sleep to interrupt LBs if they are playing with their head up and know what to look for.