You will want to begin by making an outline. You will of course want to explain what sleep is, the different sleep cycles that individuals go through during the duration of sleep, and I would then discuss circadian rhythm. You can really elaborate on circadian rhythm and how it affects people who have irregular sleep schedules, such as those who do shift work. You should also include what the suggested amount of sleep is that individuals are supposed to get every night. This differs according to age. In addition, discuss the importance of sleep.
I would continue by discussing different sleep disorders that exist. Explain the major ones by defining what they are and give examples of each one. You can easily find case studies of individuals who have these disorders.
As far as the presentation itself, the most important thing to remember is to make sure you are prepared. Know your material so that you do not have to continuously look at notes. You will also be asked questions at the end of your presentation so this will prepare you for this as well. If you are prepared you will be able to relax a little more easily.
Below is a link that contains a great deal of information on sleep. Hope this helps!