How to repair a small hardwood crack?
This looks like a crack caused by the wood drying and shrinking. Likely caused by dry winter air, although it's also possible that the wood wasn't properly dried before the bench was assembled. (Humidifying your whole house can help with cracks like this, but wood is a natural material and nothing will preserve it perfectly forever.)

It also doesn't help the way the bench is put together: ideally wood furniture should be capable of a little wood movement. When wood absorbs moisture it expands perpendicular to the grain; when it dries out it contracts perpendicular to the grain. (Wood also expands and contracts in the direction of the grain, but usually it is small enough to ignore.) The way this bench is constructed with that large surface of wood, there's no way for a little natural shrinkage without a crack like this.

Unless it's driving you crazy, my recommendation would be to stain the crack and then just leave it. You could try using pocket screws from underneath like HerrBag suggests, but if you try to close the crack tight there's a decent chance that you'll just get another crack somewhere else next winter when the wood shrinks again.

(I disagree with longneck's comment that this is caused by lack of structural support underneath: the crack probably happened next to the support because the support was holding the wood together and resisting shrinkage there. If you say it appeared in the middle of the night, that reinforces my suspicion.)