If you turn it on/off too often it can give the impression that you have a defective set up and will result in an automatic reduction in your broadband speed
Thank you redman, sunny- dave and jomlett for your prompt and helpful replies. Ours is a BT Hub 3 but there's no sleep option on it. I think we'll leave it on all the time now.
The sleep option was in the HomeHub Manager not on the hub. if you paste these numbers in your browser bar. your Home Hub Manager should show... http://192.168.1.254/