His working habits became characterised by working obsessively long hours, sleeping at most 4-5 hours a night and relying on the heavy use of amphetamines to maintain his activity levels. In 1979, his friend Ronald Graham offered a $500 bet, challenging him to go without drugs for 30 days. Erdos met the challenge, but later complained bitterly that the progress of mathematics had been held up for a month by this silly bet, because his mathematical output during this time had been zero.