Why do my torrent speeds go up and down like a wave?

I’ve noticed when downloading things that sometimes the torrents speed gently goes up, then back down, then back up… all in perfect rhythm. What is the logic behind this happening?

Does is have something to do with the seed to peer ratio? Again, I stress that this phenomenon is consistent throughout the entire download and moves far too predictably for it to just be seeders and leechers dropping off at that very moment.