There may be three reasons.
First, it may be as simple as not getting enough sleep, so try to get enough sleep regularly.
Second, you may be feeling bored and not finding meditation interesting. If this is the case, try to understand the technique and its benefits and develop more of an interest and enthusiasm. Or try doing another meditation technique, which you may find more interesting.
Third, you may feel sleepy as you slip into your subconscious during meditation. This kind of sleepiness is good, and will give you a nice, deep meditative experience.