Look at your sleeping pattern and diet, has this come about recently - or if it's just because your job is boring you'll have to put up with it. can you go for a quick breath of fresh air?
Don't sit for long extended hours. Try getting up and walking from time to time. Don't focus too much on work too if it gets you sleepy. Try texting, munching some snacks, drinking coffee or doing anything else which will perk you up.
You need to have adequate sleep at night to avoid feeling sleepy during the day. It really is as simple as that. If you have been sleep deprived for any length of time you will need to sleep extra to catch up.