Poor_ claire and Trishie certainly have a great amount of experience in prayer, so I consider their advice extremely valuable. I too think this problem may have a spiritual component. It may also be due to temporary stress, a minor change in body metabolism, or simply an indicator of a need to change your prayer pattern.
As with many problems, a multi-facted approach might be best to employ. You might try breaking your prayers up, since extended prayer while you are preparing to sleep is taxing on the small reserve of spiritual energy you have left at day’s end. Make some of your prayer in the morning, when you are refreshed and re-energized. Not all of it, but just a bit. During the day, as difficult as it may seem at first, continue with your prayer. Conclude your prayers at night. Try this for a few days and see what happens. If the problem goes away, it may be just that you have acclimated a little too much to your prayer pattern and need a slight revision.
However, if sleepiness (watch carefully for some new distraction) attacks all times of prayer, you are safe to assume that it is a spiritual attack. Take Saint Paul’s advice from Ephesians 6:10-17 and armor up! Look up a few good Catholic Spiritual Warfare prayers on the web and you will find one which suits you. Pray it will all confidence and a sense of urgency, as they are extremely effective against even demonic oppression. The attacks of the evil one may be occasional, sporadic, consistent or even unrelenting, as many of the great Saints have attested. Take heart that the Lord’s power in such cases awaits your call! You will rarely find a quicker response to prayer than during a spiritual attack. It can be instantaneous.
We know, and are confident that God is always infinitely more powerful than all of the forces of evil combined, so we take our comfort and refuge in Him. Take comfort also in the fact that evil considers your prayers to be a threat! This in itself carries three blessings: (1) You have a greater awareness of evil’s presence and design against you; (2) Our Lord is pleased with the value of your prayer, since evil chooses to oppose it, and (3) The evil one, being a coward, will flee when you resist him. Much more than a vexing concern, this has become a blessing to you.
Christ’s peace be with you.