Let awake dogs tell the truth

Good advice, but how could the sleeping ones even say anything, untruthful or otherwise?

Saying, "Let awake dogs tell the truth" makes no more sense than verbally allowing them to fib while snoozing: dogs can't talk in any case, as we all know, and so they could not tell "lies" in any alertness-state --- i.e., whether they are asleep or up and around.