Variations, Precautions and Contraindications
With care this pose may be performed by anyone unless there is a serious lower back or neck injury or a history of chronic back pain, but care should be taken in the event of high or low blood pressure, or headache. (I found two contrary references to insomnia – one saying to avoid matsyasana, one to use it as a therapeutic asana. My view is that it is a stimulating pose and should not be performed for any length of time especially in the evening if insomnia is an issue).
If the neck and thoracic spine or shoulders are stiff, this will be an uncomfortable asana as the degree of neck extension will be severe. The back should be supported by a rolled blanket so that the head rests comfortably. Alternatively, the knees could be raised and the feet flat on the floor to gently relieve compression of the spine.