The problem with cheap flights is that they can be so early. This, obviously, seems fine at the time. And then, at some point, you realise that you'll probably need to get up at 4. Which is okay, only your plan of going to bed at 10 with cocoa becomes a little bit midnight with wine.
And then, all of a sudden, your life is what it must be like having a toddler. Only with the horror of getting abroad.
Most of the time it goes all right - but we got Hamburg wrong. We should have napped as soon as we got to the hotel. Instead we wandered blearily around all morning before crashing just after lunch. We woke up at 6, groggy, went out for one drink, then lurched back to the hotel and had an early night, sleeping in so that we missed the chance to go on a boat/bus or walking tour. We walked to a bridge and then left.
At some point, more by luck than anything else, we found the giant miniature railway exhibition. Which was amazing. But we were off our heads on sleepiness so just ran around giggling. Even trains were a bit wasted on us.