The hour before sunrise is the best time to see any city. The streets are nearly empty. The light is silver and slowly turning gold. The bakeries are starting up. The fishermen are coming in. The night-shift cleaners are heading home and the morning-shift workers are appearing on the streets in twos and threes. You can hear the city breathing.
Almost no traveler does this, because almost no traveler is willing to set an alarm for 5:30 AM on holiday. We make a point of doing it once per trip — usually on the third morning, when jet lag is mostly resolved and we want to see the city in a different mode.
Cities that reward this most: Venice (genuinely empty before 7 AM, even in summer), Marrakech (the muezzin call to dawn prayer is unforgettable), Tokyo (the Tsukiji outer market by 6 AM), Lisbon (Alfama before the cruise ships arrive), Paris (the Île de la Cité before tourists), Hanoi (the parks fill with tai chi practitioners at dawn).
The other thing pre-dawn walks do is calibrate everything else about a city. The neighborhoods that are interesting at 6 AM are interesting at every hour. The neighborhoods that are dead at 6 AM are usually a little less alive than they appear at 2 PM, too.