You must be mentally competent when you sign a Power of Attorney. In other words, you must understand that you are entitling another person to handle your legal and financial interests and what this means. If there is any doubt about your mental competence, it is in your best interest to have a medical professional certify that you are competent before you sign a Power of Attorney. Otherwise, there is a risk - greater or less, depending on your circumstances - that the Power of Attorney may be ruled invalid later on, when you most need it to be in effect. Powers of Attorney are meant to reflect your competent self exercising decisions for you while you are incompetent: if you were incompetent when you signed the document, then it is invalid.
But how exactly is mental competence determined? There are a few options for determining this that you may write into the Power of Attorney (for example, you may indicate that a specific physician must make the determination, or that two or more physicians must agree on your competency), but the professional opinion of a physician is required. An Agent cannot assume responsibility of your concerns until you are determined mentally incompetent. In some cases, it may be necessary for a court to decide the matter of mental competency, according generally accepted standards.