Since you invariably falls asleep during lectures, and not when you are doing any other actively engaging/interacting work, I presumed you are not overtired, or lacking of sleep. Most likely you lack interaction. Whenever you talk to someone, you will not fall asleep. So when the lecturer are talking, it is very often I take down notes. And there are two kinds of notes - just like different ways of talking to the lecturer.
If I already understood what he said: you can often see me dropping down QUESTIONS which I want elaborate further - likely to be on my own, or with hime/someone else.
If he covered some topics / area which I am not familiar, I will try to understand at a high level (if I were to zoom in understanding every word he said - it is not fruitful), and perhaps summarized as one word or two what he said - which I will explore further after the lectures.
Through these two types of notes, often it can sometimes be overwhelming as I try to concentrate listening + writing at the same time - strike a balance for yourself.
Another way? In the past I used to use my handphone, mp3 recorder etc to record the lecturer teaching, but that required you to seat in front.
SITTING IN FRONT does help too - as I find myself more likely to interact (with lecturers) through asking questions.
My past lecturer also encourage questioning - intelligently posed, which will score points. And so all of us are encouraged to engage in actively making remarks, asking questions to further our understanding. Once in a while some chocolate bars are offered for the highest numbers of questions asked, or best questions posed (some times very subjective, so difficult to judge).
I used to have lectures at 2.00pm too, and that is after lunch hours. As I invariably will fall asleep after a heavy meal, I did TWO things:
- I jogged at 12 noon under the sun in the stadium, or you can go swimming. All these is to build up the short period of alertness after exercising.
- Since you don't eat before exercising, that's good. And after exercising, have a small meal - 80% full usually will mean sleepiness after a short while, but 30% to 50% full usually I can stay awake for at least 4 or 5 hours. Many times I can skip lunch altogether, just to make sure I stay awake throughout the entire lectures.
Another new way for the electronic geek :
Recently, since I had a new Nexus 7 7" tablet with me, I found it extremely useful. I was listening to a lot of different talks (in a Conference), and many times the technically advanced speaker will mumble something which I don't understand or know of. Instead of writing down some keywords to be explored later, I entered into Google the keywords, out came the many search results as answer INSTANTLY. It helped me to understand IMMEDIATELY the outline ideas, but more important - I just enter "star" button and this search results is remembered as a favorited page. The list of the favorited pages exported out as HTML can be easily imported anywhere to be explored further just by clicking.