Hi Twinmoon,
Although my colleagues have given superb answers to your query, let me add my 2¢. (See it is all about the money) Hopefully, I will cover some new ground.
When you work the unpaid time, you can really hurt not only yourself, but also your co-workers and the company.
Hurting yourself:
First of all, it is a job. You are hurting yourself by leaving money on the table. I know money is not your number one motivating factor, but it is a factor in our world. Right now you are young and single (assumptions) and money is not all that important. You have enough to survive, have some fun, etc. I am married with 2 kids. The equation changes when you have other people to worry about, as I am sure it will eventually change for you. (I know you don’t want to think about it now; I know I didn’t.) Take the overtime pay and put it in the bank, IRA, mutual fund, etc. Nothing will change now, but you’ll thank yourself later.
Also, you are setting yourself up for a psychological fall. You are not getting paid for all of that overtime, but in your mind, you are accumulating something, call it goodwill, karma, kwon, whatever. Your basic human sense of fair play will tell you that this company now owes you something. Believe me, the company will not see it that way, or will have amnesia (or the boss that you did all that extra work for is reassigned and you get a boss who doesn’t know you from Adam.) You will be called into the office to be chewed out about taking an extra 5 minutes for lunch, you won’t be allowed to leave early one day, you have to come in on the Saturday of your niece’s birthday party, etc. You will feel wronged by the company, and usually only bad things happen from then on. Let them pay you to even out the score; it is pretty hard to track karma owed.
And as Golom pointed out, what happens if you get involved in something else like sports, significant other, etc. where you can’t or don’t want to put in that extra unpaid overtime; you will either have to work extra hard the hours you are there, or start getting bad performance reviews because “You used to be able to get it all done working X hours a week,”
Your Co-Workers:
I won’t comment on if your co-worker(s) “really wasn’t doing anything.” Just do the math. Suppose you work 40 hours a week, and put in 5 hours unpaid (untracked) OT. Your co-worker puts in just 40 hours. Now that co-worker has to work harder than you just to keep up. Of course it seems like they look bad, whether they are bad in reality or not.
Now, as KenCunningham said, what does “happen when you’re on holiday?” Now your co-workers not only have to make up your regular 40 hours, but they also have to make up the phantom 5 hours! I’d start resenting you, too.
The Company:
Now, this ground has been covered quite well. I’ll just add that in addition to masking any deficiencies that the company might have in it’s scheduling, you are also setting up your replacement to fail. They might not be as altruistic as yourself, and find that they cannot complete the work in the time that you did, and start be pressured by management, resulting in an unpleasant work environment. Much more often than not, it is a bad worker that can poison a work environment, but I have seen it where a “good” worker can do the same.
Also, I know this probably does not apply to you, but I work at a place where we bill our clients for time worked. If it isn’t tracked and logged, then we can’t bill the client, and the company loses money. They tend to look down at things like that.
Summary:
Hopefully I haven’t come across as too cynical – just experienced (and too much Dilbert!) It’s great that you like you job so much that you’d do it free. Count yourself lucky. I know I count myself lucky in that department as well. But I also count myself luckier that I get paid to do this! Money is not my main motivating factor for the work I am in – I was making more money at my last career. But, it certainly is up there when I have a family to think of.