We'd definitely need more info on which aspect of STEM to give you a more detailed answer. For example, I started out in Chemistry in the lab, the work week there was pretty standard 40 hour week with almost nothing outside the 9-5. I transitioned from that into IT, my first job was part support and part training, again fairly standard hours and little to no extra required.
Next came full IT support work, that's when things changed. Many of the upgrades / system changes can't be implemented in normal working hours because the business can't afford the downtime, so overtime/evening/weekend work became a little more normal. Some of the positions I've had in that field (IT Support) require you to be part of an on-call team, so at least one week a month you have to be available 24/7 for problems as well as your usual work week. I've also had colleagues who have supported more critical systems who work almost every weekend to implement changes or upgrades, and because of their specialised knowledge are on-call almost 365 days of the year.
The workload is similar - some companies and positions the workload and stress levels are fairly standard (comparable to most office jobs), others support more critical processes or higher stress (think an engineer who maintains a nuclear reactor for example, or a forensic science technician who has to deal with the aftermath of crimes). So it all depends on which career option within STEM you want to aim for.