Keep Left?

Whilst commuting to work over the past few weeks a thought has struck me.

In the morning my fellow commuters and I follow the simple instructions we are issued and we 'Keep Left'. All walking as near to the left hand wall of all corridors in the station.

When I do the return journey in the evening everyone seems to consciously flaunt the rules and walk as near to the Right as possible.

I have had reason to do the journey in the middle of the day a couple of times recently; everyone is still walking on the Left.

So when does the flow change? At what point of the day do we start to rebel?