Final answer:
The priority nursing intervention for this client is decreasing environmental stimulation to reduce agitation and promote better sleep.
Step-by-step explanation:
The priority nursing intervention for this client is decreasing environmental stimulation. The client is exhibiting symptoms of mania, which includes hyperactivity, poor sleep, and poor grooming. Decreasing environmental stimulation can help the client to find a calmer environment, reducing the risk of agitation and promoting better sleep. This intervention can also help to decrease the client's overall hyperactivity.