how intense is the work load in a stem career and what are the hours worked per day?

A lot of the intensity depends on who you work for.

I work in an envirnoment that's 24x7 so you can be working at any time, any day (holidays included).

Other companies literally close at 5 so the pressure to work 'out of hours' is almost non-existsent.

I'm lucky .. I work about 50 hours a week; however, I don't usually get called in the wee hours of the morning (anymore).

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.

Hi!

It varies enormously, so unfortunately it is hard to give much of an answer.

I work a bit under 8 hours a day, five days a week, on average. I try to hit about 8, but I'm usually a little short. This has been completely accepted so far; my manager and coworkers are happy with the amount of work I get done. Google is very much focused on "complete your projects," and try not to plan projects that will require you working unreasonable hours. Though, even at Google, at least on certain teams at critical times, people have been known to spend much more than 40 hours a week working.

In other jobs it can be entirely different. There is a stereotype that software engineers working at a startup might work 60, 80, or even more hours per week (twice as much as me!) because there is so much to do and you need a product before you can really start getting paid. The game industry, too, is known for this kind of thing. But I'm sure there are exceptions to both.

Outside of software engineering there is also huge variance, but I don't have any experience there to speak from.

Some people talk about "work/life balance," as in balancing the amount of time, energy, and so forth that you put into your work versus your personal life (alone time, chores, friends, family, hobbies, and so on). I cared about finding a place with good work/life balance when I was first looking for a job, so before I accepted the job at Google I made certain to ask everyone I could about what the hours tended to be like. I decided not to join an interesting team, in fact, because the manager couldn't promise that I'd never have to work nights or weekends, and instead I chose another team whose manager said "If you have to work extra hours, I haven't done my job."

If you are also concerned about this, and I think it is a good idea and healthy to be, then as you start to focus in on more particular areas ("STEM" is pretty broad), pay attention and ask people involved in that field about how much people work. For example, as I mentioned, game development is known for its hard hours, so if you started being interested in that way, try to talk to people involved in game design and see if they all report more work than you think feels reasonable. If they do, then consider looking for other, similar fields with better work/life balance.

Does this make sense? It's hard to plan these kinds of things about your life far in advance (I had no real concept of what working 40 hours a day would feel like, as opposed to taking classes all day and having homework, for example), but it's good to start early so you can spend a long time building up a picture.

It literally depends on the career path you take, the company you work for, and project that you are working on. I'm sorry for such a bland answer but STEM covers a lot of areas, disciplines, and industries.