When you say "sleep mode", do you mean like Z80 having a HALT instruction which waits for an interrupt, or do you mean entering a very low power state like a laptop put to sleep?

6502 emulators can easily detect dumb endless loops, and that can turn into what's basically a Z80 HALT.

Nitpick, the laptop mentioned is actually using a 65C02 which does have STP (stop clock) and WAI (wait for interrupt) instructions.