Entry 471 - The scenic route home

When I go to the gym I usually go on the road from Vauxhall past St Thomas', down York Street and round the big roundabout at Waterloo where my gym is at on Stamford Street. When I go back, I like to go down the river, the view is nicer :) Here you can see Westminster and the London Eye.

Can someone tell me why some night the street lights are on by St Thomas' Hospital and some nights they are switched off?