Is it true that worship is only possible in a sitting posture? why we can't worship in sleeping posture?
This is scripture.
Brahma Sutras IV. I. 7-10. Verse 7 reads: "(One should meditate) in a sitting posture; because (in that way alone) meditation is possible."
Gita VI. 11-12. "Having established in a clean spot a firm seat...there sitting on that seat...he should practice yoga..."
Svetashvatara Upanishad II. 8. "Placing the body in a straight posture, with the chest, the throat, and the head held erect, making the organs enter the mind,..."
The mind will tend to sleep when done is a sleep position. If you try to do so standing, part of the mind must be kept aware of balance, full concentration is not possible. When done is a sitting position as described above, the mind is able to lose it's awareness of the body and the mind is able to concentrate all of its power internally.