relax可以与一段时间连用，但不能用反身代词作宾语。所以， Don’t work too hard. You can relax yourself for a while.应该改为： Don’t work too hard. You can relax for a while.下面是一些实例：
一、席玉虎主编《英语常用词用法详解大词典》p.2552：relax不是反身动词，不能说relax oneself。例如：
我经常洗热水澡来放松自己。
误：I usually relax myself by taking a hot bath.
正：I usually relax by taking a hot bath.
Let's stop working and relax for half an hour. 让我们停工休息半小时。
二、《张道真高级英语词典》p.1245：
Why don't you relax for a few minutes? 何不休息几分钟？