Sleep is an interesting thing. For some creatures, it is a an undesired necessity. Children, for example, will refuse to go to bed, no matter how tired they truly are.
Spike is a child and it doesn't matter how much he's yawning or how much difficult he has keeping his little balance; he is not tired and he will not sleep.
"Bedtime," says Twilight.
"Not without a fight," says Spike.