There are several different ways that you can get bedhead waves. However, it is important to realize that the type of hair you have will determine the results. Not all hair types are capable of doing the same thing, therefore you will need to work with products that you know work well for you.
One option to get permenant bedhead waves is a body wave perm. This perm, if done properly, provides your hair with a soft wave. It's great for those summer months with humidity becomes your worst enemy and, have no fear ladies, it won't leave you looking like a poodle. Just make sure that the stylist uses larger rollers so that you get the soft wave look instead of spiral curls.
If you just want waves for one day, wash your hair at night and then apply a textured spray, such as Garnier's Strong Curl Shaping Spray Gel. Make sure that your hair isn't too wet, as this will prevent the hair from drying by morning, which will prevent the curl from properly taking shape. If you have thick hair, you can part the hair in sections or, if you have thin hair, make one single bun. Twist the hair before forming the bun and then secure. In the morning, shake it out and scrunch it. If your hair is still damp, scrunch it while blowing it dry. You can also take a different approach if you want smaller waves in your hair. Take small sections of hair and twist it before forming small buns and pinning it back. You can do this at night and sleep with them in or you can do it in the morning and then blow dry the sections dry. Either way, the end result will be loose, natural looking waves. Just be sure not to pin the ends improperly, as this will result in fish hooks.