When you are offered an assignment, you are probably most eager to take it. After all, a new assignment means income. But sometimes it is better for yourself and the client to refuse an assignment. Therefore, ask yourself the following questions before you say “yes".
As a ZP'er you prefer to cram 48 hours of work into a twenty-four hour period. But this can be at the expense of your health and the quality you deliver. Therefore, always make sure you make a realistic schedule before taking on an assignment. Keep in mind that things can go against you, so keep slack in your schedule. If you don't get it done in the time the client gives you, be honest about it. To yourself and to the client.
You can resolve to live on five hours of sleep for three nights, but that will not help your condition or the quality of your work. You run the risk of finishing the work or making mistakes due to time pressure and fatigue. Small chance then that the client will ever come back to you. Don't have the time? Then don't take the job. But do try to help the client by, for example, putting forward a fellow contractor with whom you have had good experiences. This way, you do the client a favor and preserve the relationship, which may yield work in the future when you do have time.