Do people working at a Big 4 firm sleep?
Of course we sleep, just not much when it's busy season.
Let me just preface that technically speaking without a doubt, everyone's sleep schedule is uniquely different. But if you work in a Big 4 audit team (which is in most cases), then you will most likely arrive to work at the same time and leave at the same time. Which in turn also means you will have similar sleep schedules.
It's busy season for all you auditors out there. So, this just means you will be busy and could possibly work on the weekends. Perhaps not every weekend, but you can bet on some weekends.
Working on the weekends depends on many factors including the team you are on and what your audit engagement has budgeted for the job.
From personal experience, from January to March, there have been some jobs where I've had to work every weekend for a month. On other jobs, I've never had to work a weekend. If you are an associate you are essentially at the mercy of your senior or most times manager.
The industry wide standard practice is that during busy season, you work a minimum of 11 hour days. That is a minimum, so it is very likely that you will go over. On a normal day at a Big 4 audit client during busy season, you could get into the office anywhere from 7AM to 10AM and leave as early as 5PM, but as late as 2AM. Honestly, a lot of it hinges on your senior and what kind of schedule he or she is going to set based on what the manager expects.
If you are a first year associate, my recommendation would be to ask your senior what kind of hours you will be working. You won't get dinged for asking this basic question. This way you are not left wondering when you will leave and basically working until he or she says you can leave. At the very least, you will know ballpark when it is time to pack up and call it a day. In auditing, as I quote a partner, "If you really wanted to, there is enough work in auditing to work yourself to exhaustion".
Remember to pace yourself, do your best, and you will see light at the end of the tunnel.