Which sensor are you using?
Usually the sensors will have capabilities where you can set the output data rate.
You can set the rate on your sensor and put less load on the BLE chip, which will optimize the system power consumption.
Also, some sensors have FIFO available. So you can store a lot of data to the sensor's FIFO, before you pull the data.
The sensor datasheet should help.
This approach is probably the best because there are some limitations to the timers on the BLE chip. (app timer : 1 sec; fine timer: minimum 12.5ms)
But there's another timer that you can use at a resolution of 1.25ms.
This post will help with setting up the timer if you want to use timer.