Using Macbook Pro and Monitor as TV decreases macs life?

I have a macbook pro early 2008, and I am debating buying an external monitor. If I watch movies or TV with my macbook pro on this monitor will I damage my mac? For example, if I watch tv on my mac before I go to bed, and I fall asleep leaving my macbook running all night will my mac over heat or become damaged? I don't plan to have it running all night, but this may happen if I am not careful.

Also, should I leave the mac open when I am using an external monitor. I believe macbooks vent through the keyboard, and I am afraid folding the screen down will cause it to over heat, especially if I am using it a lot. Is there any other risk besides over heating (eg. logic board, video card, hard drive, ram)? I am just not sure what could be damaged by letting the computer actively run shut for long periods of time in addition to watching videos/tv for extended periods of time.

I kind of asked a lot, but any input would be great. I have atttempted to find a similar question, but I have had no luck.