Tandra is the first of the four states of mind that a yogi attains during meditation. It is followed by yoga nidra, yoga murcha (swoon) and samadhi (state of complete concentration).
Yoga tandra is ususally achieved very early on in meditation. Prana starts to work in the deep areas of internal organs and the body and mind become very relaxed. The yogi begins to feel sleepy, eyelids are heavy, and the mind is not able to think intentionally any more. It is a state of inner peace, developing the state of yoga nidra, that follows after transcending tandra.