In August the temperature is around 40º (104F), but by midnight it drops to about 30º (80F). This is when the locals start to go out and they stay out till 6am. It's worth staying up to sit in an outdoor cafe at 4am, particularly near the river, and definitely worth having a big siesta.
If you go the main Post Office, make sure you get a ticket from the machine on the right-hand side at the front of the main concourse before queuing. We were in Seville three weeks ago and did not notice the machine - I ended up having a heated exchange with a girl I thought was pushing in front of me after about a 20-minute wait!
Many visitors take advantage of the siesta to take photos or see more of the city while it's quiet, but don't do it. Don't wander the back streets of Santa Cruz or any place with few people during siesta (2:30-5pm). It's better to follow the Spaniard's example and use the time as it should be used: for relaxing, eating lunch, taking a nap and then heading back out at 5pm.
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