Generally, TDMA is used, where all nodes have time slot, therefore nodes can sleep until there time. Other techniques like LEACH, group the nodes into clusters so that the nodes can sleep and wake up intervals just to sense their environment. The head of the cluster does not sleep because it is responsible for collecting sensed data and forwarding it to the sink through other Cluster Heads (CH).

Therefore:

1)You can group nodes and group head does not sleep while others sleep,

2)You can allocate time to each node, hence child nodes knows when its parent is asleep in order to avoid transmitting at such times.

i have to solve this using the idea of coverage. if the sensing region of a sensor node is covered by other sensor nodes, it can go to sleep.

so how to calculate if the sensing region of a sensor node is covered by other nodes.

Try Data correlation