I use to meditate properly in a sitting posture, But recently my physical health got worse and it restricts me from sitting for a long time. hence I started meditating lying down, But I have observed that it is difficult to maintain concentration in lying posture due to sleepiness as compared to the sitting posture. how can I practice meditation in lying posture with full concentration.
Shavasana is the most famous way of meditating while lying down. The only restriction is that one may not lie in left or right side. The posture has to be facing the sky:
To perform Shavasana, lie on the back with the legs spread as wide as the yoga mat and arms relaxed to the side. The eyes are closed and the breath is deep with the use deergha (long) pranayama. The whole body is relaxed on the floor with an awareness of the chest and abdomen rising and falling with each breath. During Shavasana, all parts of the body are scanned for muscular tension of any kind. Any muscular tension the body finds is consciously released as it is found. All control of the breath, the mind, and the body is then released for the duration of the asana, typically 20–30 minutes.
The asana is released by slowly deepening the breath, flexing the fingers and toes, reaching the arms above the head, stretching the whole body, and exhaling while bringing the knees to the chest and rolling over to the side in a fetal position.
But I have observed that it is difficult to maintain concentration in lying posture due to sleepiness as compared to the sitting posture.
You don't need to worry about this. In fact, you can use such condition as meditation as well! From VijnAnaBhairava Tantra-
When sleep has not yet fully appeared i.e. when one is about to fall asleep, and all the external objects (though present) have faded out of sight then the state (between sleep and waking) is one on which one should concentrate. In that state the Supreme Goddess will reveal Herself. Verse 75.
Now, how to concentrate?
For concentration there are many ways one has been described here. Another is repeatedly direct mind towards your present condition as whatever you practice repeatedly it becomes habit of mind whether concentration or distraction (Watch this). So, for concentration only you can do is you can put right efforts. You can also witness flow of your breath while lying down. When you are distracted concentrate again. repeat this concentration repeatedly.