Quote Mick Gledhill="Mick Gledhill"When I went to Seattle for two month a few years back it took me a good week to get my body clock back in the groove.
It's alright going over, it's coming back that's a pain.'"
Always is, mate.  Westbound you get over reasonably quickly; Eastbound is the killer. 
Although if you want REAL jet lag, try doing MAN-LHR-Christchurch, or Melbourne, in cattle class with only a two hour transit in Singapore.