Greetings Ashley
This thread,
An idiots Guide to Dhamma Diagnosis is a pretty good place to start and get a sense of where you are on the path.
Why would you want to know the whole path and everything? Because without a the constant company of a good teacher (who's advice you actually take) around the corner, it really helps to know where you are and where your going.
Sleepiness can be a direct response to either to much relaxation (lack of attention) or too strongly fixing your attention for periods. When you strongly fix your attention, in the short term you can get to really good places, but over a day of mediating for 12-14 hours or over a week, this becomes less affective. Ask yourself next time your concentrating, how much tension is there? In your neck, forehead or anywhere? You might notice a subtle desire to see the clarity of the object more clearly, along with a push to make this object more apparent! This object could be light, space, bliss, equanimity or the breath. With the breath, in insight practice, I used to have a lot of denial about the clarity of my perception of the breath, about accepting it the way it is, I didn't realise how hard I was trying. And how that hardness was actually interfering with the practice. When doing samadhi/concentration practice (anapana), and modifying one experience, modifying because there is a direction for the meditation towards pleasant states. Such as by making ones self more relaxed at the start, and then focusing on pleasant states. Or by trying to become more attentive and absorbed in the object, to give it clarity and increase pleasant states. In the practice of samadhi we need to be careful about exerting, as there is a tension connected with this.
A good model is relaxation, stability and clarity. If you want clarity you need stability and relaxation. If you want stability you need relaxation. But relaxation must be balanced against the forces of laxity and excitation, using relaxation and clarity/attention.
So for example, clarity of the object is improved by actually staying with the object, and not missing it for brief moments of distraction. If your getting sleepy, you can use that quality of focusing harder to see the clarity of the object. But if that's your main or only tool for developing, rather than a tool you use for balancing laxity and excitation, then you will have trouble developing your concentration. Be that momentary or fixed! The best way to build the full-house of concentration is on the foundations of relaxation, then with the walls of stability and lastly with the roof of clarity.
So maybe your sleepiness is nothing! But good samadhi makes you wakeful through the night! and if you continue to get sleepy this is because of the things i have stated that maybe within your practice. Balancing laxity and excitation is a continual, process throughout the development of meditation, including insight. But in insight it is balanced more with mindfulness, but the seven factors of enlightenment point to the choice of a quality of interaction, rather than allowing the 7 factors to become present through continual mindfulness and allowance. As is usually done in dry insight.
All the best and good Luck, from neem.