Hi,

you can work . GRA position is different than the other work given in the US university . for example.  F1 student enrolled in microbiology or any science  Ph D program or master degree program  have to work extensive hour in the lab performing reserach ( sometime 8AM to 10 PM  or even more). if  IF Ph D student doesnt allow more than 20 hour a week in the lab how can they finish their project within 5 year ? GRA work is totally different and they dont pay by hour  it is paid on the basis of semester or based on the yearly GRA grant. I am not sure your position . when I was in the graduate student I used to work more than 40 hour a week in the lab and never had any issue.