Calibrating a monitor with varying ambient light?

My setup is in a room that varies in light throughout the day with natural and artificial light and then is different at night without the natural light. I use the computer during all these different periods. I am looking at getting the Dell U2412M and possibly a Spyder4Express.

Is calibrating a monitor with changing light pointless, or is calibrating a monitor without it possible, or not worth it? I am an enthusiast photographer, so it's not the end of the world so I don't need to the pro standard, just good enough.