A source told British newspaper the Daily Mirror: 'Robbie suffers from crippling anxiety and has been worried about the lads' forthcoming concerts. A pal suggested Bikram yoga to refocus his pent up energy and handle stress more effectively.'
Williams, who is in the band with Gary Barlow, Jason Orange, Howard Donald and Mark Owen, has apparently been going to Bikram yoga sessions for the past three months, which sees him meditating in a room heated to 40 degrees centigrade.
The source added: 'After a couple of sessions he was hooked - and sweating like a wildebeest. While the others have stuck to the gym or pool, Robbie has been using meditation and yoga to help his stage fright and to keep sane on tour.'