It is sometimes necessary when the situation dictates to increase working hours to achieve a shortened schedule. However, numerous studies can be found to support that Overtime results in lost productivity. There are other factors that affect productivity, but just to address the topic of Overtime, for the moment they will be ignored. This productivity loss set of data is from Applied Cost Engineering, Clark and Lorenzoni, Marcel Dekker, Inc., 1985.
As both hours and number of days worked increases over 5 days and 8 hours, productivity declines. 5 days and 8 hours is considered the norm = 0% productivity loss. Any increase in hours or days above this norm reduces productivity. All values approximate and % loss is loss of production on ALL hours worked.
5 days and 8 hours = 40 hrs @ 0% productivity loss = 40 hrs productive
5 days and 10 hours = 50 hrs @ 7% productivity loss = 46.5 hrs productive
5 days and 12 hours = =60 hrs @ 12% productivity loss = 53 hrs productive
6 days and 8 hours = 48 hrs @ 3% productivity loss = 46.5 hrs productive
6 days and 10 hours = 60 hrs @ 17% productivity loss = 50 hrs productive
6 days and 12 hours = 72 hrs @ 25% productivity loss = 54 hrs productive
7 days and 8 hours = 56 hrs @ 7% productivity loss = 52 hrs productive
7 days and 10 hours = 70 hrs @ 20% productivity loss = 56 hrs productive
7 days and 12 hours = 84 hrs @ 28% productivity loss = 60.5 hrs productive
Not only does overtime produce lost hours, but the cost of the overtime hours increases. Hours over 8 might cost 1.5x normal rate. Days over 5 might cost 2x normal rate. Increasing days and hours rapidly balloons the cost of completing the work. However, if absolutely necessary to meet unusual schedule demands, the cost vs time to complete work can be modeled for each scenario and the least destructive option (whether that be cost constrained or time constrained) can be agreed upon by all parties. The best choice is always that which requires the minimum added cost to achieve the restricted schedule.