No it shouldn't disconnect. It may be connected to a burst of noise.
FWIW last night (I think for the first time for me) around 1am the PPPoE session was kicked off and it couldn't reconnect until around 2am.
Neither my router nor the VDSL modem were rebooted and the VDSL was sync'd for more than 4 days since I rebooted everything (by mistake, kicked socket!). Right now my router's system uptime is 3days 6hrs, and my PPPoE session uptime is 22 hours. The modem's system uptime is 3days 6hrs and the VDSL uptime is 283542 whatever units that number is in.
Guess BT doing work on a RAS box?
James BT Infinity 2 19/09/2012 - Estimate 44.6/6.5 - Install 52/12 - Actual 46 / 8 Mbps
13 years of broadband - ntl:(512k/1M)/BTbusiness(2M)/Metronet(2M)/Bulldog(8M/16M)/BE(16M)/BT FTTC(50M)