In the silent movie “Modern Times,” Charlie Chaplin plays a factory worker turning bolts at such a frantic pace that he suffers a nervous breakdown.

Some young Chinese tech workers see themselves as 21st-century versions of the Little Tramp, working the dreaded “996” schedule—9 a.m. to 9 p.m., six days a week.

Just ask Galvin Guo, a 33-year-old programmer in Beijing. For much of his 10-year career at four telecommunications and internet companies, he worked the 996 schedule.

At one job he suffered from frequent colds and rhinitis, he said, and at another he struggled with stomachaches and depression. Arriving at the office every morning caused anxiety, he said, and he was too stressed to accept social invitations.

Now a manager at a new company, Mr. Guo said he has been under pressure to make his team work longer hours. “996 is an unspoken rule at tech companies in Beijing,” he said. “The boss would think it’s his loss if you don’t follow it.”