How can I tell if I'm depressed, or just tired?

Well, try to get some sleep first instead of playing online.

I hit a similar wall 2 weeks ago and you just need to stop and get to bed and get some much needed rest. I started forcing myself to go for a quick run every other night and that did wonders for blasting away the stress build up. Too much work and little play will push you towards feeling depressed. Cut down on the work or spread out the load so it is less burdensome.

Get a good night’s sleep and then see how you feel.

It sounds like you just need more sleep, but overworking yourself can also make you depressed. What you really need is a day off to get a good night’s sleep, get some exercise out in the sun, eat a healthy meal or two, and spend some time having fun with friends or family. Any of these things will make you feel more healthy and less tired, and can also lift you out of a small depression. I’ve found that when I’m depressed, the things that make me stay depressed are work and sitting around being depressed. Sunlight, exercise, healthy food, and some social time will really make you feel better.

If you have a genuine concern, get evaluated. A trained technician can administer and interpret an instrument such as the Beck Depression Inventory.

Try to get some good night’s sleep.
Don’t overburden yourself with work. That might be your real problem.
If you don’t feel any better after a few days, then I would seek advice.

Here is an article from webmd.com about the signs of depression. After reading it, if you recognize yourself as having several of these symptoms, you probably should get evaluated as @dpworkin suggests.

I agree with everyone else. Try taking a day off and getting a good nights sleep. Another sign of depression is no longer enjoying things you normally enjoy. If after getting a good nights sleep, you still feel down or have any other feelings of depression, you should make an appointment to discuss it with your doctor.

Here is a self-screening test for depression. Take it and find out what you need to know about how you are feeling.

In general, I am of the opinion that mental health issues are too important to trust to self-screening and other such pop-psych entities. If you are not too worried, you are probably not very depressed; if you are upset and your feelings are interfering with your enjoyment of life, get serious and see a pro. I suggest you not mess around with Mr. In-Between.

You are probably tired. Every time we feel down and tired we shouldn’t yell depression.

I felt so tired I couldn’t function, I talked to my doctor and he ran tests. It turns out I have a thyroid problem. If this persists, you should talk to your doctor. It might be depression or a sleep disorder or thyroids or any number of things.

I agree with @dpworkin particularly since fatigue and sleeping too much are symptoms of depression. But sleep as much as you need to, it may just be that you are tired, but if it continues then you probably see your doctor because your fatigue could be caused by a physical problem too.

Can’t fatigue trigger depression?

Work-related exhaustion is exogenous; depression is endongenous and defies reason.

Speaking as a layman, I’d recommend that you take a break from work—even a long weekend would be a good start—and get some R&R. If your mood and stamina improve, you might have your answer. If not, clinical depression might be a possibility.

This is a tough one to answer. You feel sleepy; which might mean that you are, indeed, sleep-deprived because of your schedule, or it could be a symptom of clinical depression.

I suggest you go to the allaboutdrepression where you can get a very good understanding of depression, its causes, symptoms and treatment. You should be able to decide whether you need to consult a professional for a more thorough assessment.
If you are still unsure, go ahead and consult a clinical psychologist or other mental health professional. You are better to err on the side of caution.