I’m a midlevel M&A/securities associate at a biglaw firm. Lifestyle is comparable to my IDB friends at associate/VP level, though not as bad as the analyst days. As others have mentioned, a great deal of it is downtime - especially in banking, where’s there’s generally more of a face time requirement - and there is a great deal of unpredictability in actual work flow. I don’t really know how to say how many hours I work per week. Emails began around 7 am and stop around midnight, so I am generally up past 12 doing stuff most weeknight. But I wouldn’t say I work 16 hours a day at all.
I think it’s more accurate to say professional services require extreme availability. It’s the existential dread I feel when my phone vibrates on a weekend, not sleeping for four days, and inability to make any sort of firm commitment to my family and friends are the true stressors in this line of work, not sitting at a desk 12 hours a day.