I always get much slower download speeds at night, quiet often several hundered kbps slower, unfortunately there isn't a great deal you can do about it.
The common problems with low download speeds in this order are:
1. failure to open ports for peer incoming connections (the "port forwarding problem"). You need to set a constant port for incoming connections in BT client, then set the host firewall, and if you have access to a router (you do at home), you set its firewall or port forwarding properties. It differs for routers (I use USR 9108, and its "virtual servers" have to be set and moreover, you have to use a static IP and not DHCP (re-set the range of the DHCP server so that you have a few addresses to be used as static IP addressses).
2. failure to set the upload bandwidth appropriately. If you are using a BT client that does not do auto-speed control (like Vuze Azureus, which I recommend to everyone; I was a uTorrent client user until recently), then you need to determine your ACTUAL (not THEORETICAL) download speed using a speed testing service on the Internet, and then set your upload speed from 50-70% of that value. You then get the highest DL speeds at hat.
3. Your ISP is throttling your bandwidth after seeing your upsurge in bandwidth use. Shut down all your torrent and streaming activity and visit the Glasnost site to determine if you ISP is affecting your bandwidth. (EDIT: I neglected to add that you should set your BT client to use encrypted protocol transmission, which typically prevents your ISP from sniffing packets that are torrent data.)
Follow this checklist.
If you have done all that, then you just have to consider the possibility that you were connected to s super-fast peer last night. Much of the time the peers you connect to may have slow upload speeds.
I think it is because you need other people who have that torrent downloaded, need to be connected to the internet and using their computer, but they might not be on their computer. It might also be that lots of people are downloading that torrent at once.
There is a Bittorrent accelerator here which might help: http://www.freedownloadscenter.com/Network_and_Internet/Misc__Networking_Tools/BitTorrent_Turbo_Accelerator.html