Jason Fried on the hours of the workday:

There are lots of ways to carve up an hour. 10 x 6. 15 x 4. 30 x 2. 45 + 15. 20 + 20 + 20. The key is not to carve it up. And when you stack it up – one full hour after another – you really see the compound benefits of uninterrupted time.

Managing time is managing interruptions and vice versa.