It depends on how different. Day length affects more than just how long the periods of light and dark are. Let's assume a very slow rotation, and a very long day, maybe 1 month light, 1 month dark. The light side would get much warmer while the dark side would get much cooler. At the border between light and dark sides, you'd expect to see powerful storms as the hot moist expanding air moved into the cooler side and had to lose moisture. Any life on a planet like this would have to adapt to surviving in both hotter and cooler temperatures. We can take this to an even worse extreme and look at a planet like uranus. Uranus's axis is tilted at 97.7 degrees, so even though it rotates, the day/night cycle is controlled by it's rotation around the sun. If the earth were tilted like this, then we would 6 months of light followed by 6 months of darkness. Life would either have to keep moving to stay in the moving temperate twilight zones or adapt to hibernate.
If we assumed a shorter day/night cycle, I think the changes would be less significant. Lets say we had a 4 hour day. ( There has to be a speed where things get thrown off the planet, but I don't think 4 hours per day is fast enough ) A 4 hour day would have 2 hours of light with 2 hours of dark. My guess is that circadian rhythms would be less important. Temperature changes from light to dark would be less significant than currently.
It's an interesting thing to think about because the Earth's day is slowly getting longer as the moon exerts a tidal force on our planet, slowing our rotation and escaping further into space, but this change is so slow I doubt it will ever become a problem for humans.
