Nah it's 100% safe leave a laptop on for weeks at a time with no damage, just take some precautions.
1.) if its a modern laptop (Centrino or Mobile chipset) then the processor with auto scale speed (power consumed = heat)
a. Make sure you set the laptop to "longer battery life" under power management (get to it though screen saver tab under display properties
2.) Have your screen turn off when the lid is closed, the display drains more power than anything else on a laptop (well maybe the Hard Drive unless its SSD and what your doing, but in a normal/web browsing/Text writing situation)
a. Once again you can do this under power management under display properties
3.) Last if you know the download will be done by a specific time then you can auto shutdown your pc at that time, I do this when I'm downloading stuff I'm not proud of
a. to do this click on START -> RUN -> type "Shutdown -i" without the ""
b. for computers type 127.0.0.1
c. type = shutdown
d. display warning for X time where X= when the Download will be done in seconds so 4 hours would be (4*60*60 = 14400)
e. to abort the shutdown for whatever reason just click START => RUN => type "Shutdown -a" without the ""
anywho thats my 2 cents, I've been downloading
porn errrrr
history documentaries for years with no problems on a laptop.