[Dovecot] interesting stats pattern

[Cor Bosman]

Hey all, im experimenting with dovecot stats service, and graphing the result. My initial results are kind of interesting. Check out this graph showing connected sessions and users:

http://grab.by/dReu

At first I thought maybe one of our 35 imap servers was having issues sending data, but all individual servers show this patters. Here is a bunch of individual servers:

http://grab.by/dReC

Anyone have any idea what could cause such a pattern? Maybe dovecot does some cleaning up of idle sessions at specific intervals? Or maybe our loadbalancers do, or the imapdirectors.

regards,

Cor