Overnight crashes

Hi,

For the past few days I experienced crashes that happen only overnight when no one is around.

While data comes in slower overnight for some reason we mainly get the following erros when back in the office the next morning:

Visualize: Bad Gateway
Visualize: Gateway Timeout

nginx log:
http://pastebin.com/9EZU5U0w

Anyone who has experienced this and could help me troubleshoot?