A siesta is a very bad idea.
Please see this post (from Clive):
CO2 is THE most important ingredient for making the sugar. Without CO2, shining a light on a plant is about as effective as boiling it in a soup. Plants produce what is probably the single most important enzyme called Ribulose-1,5-bisphosphate carboxylase oxygenase, normally abbreviated to "RuBisCO". All carbohydrates on the planet are made possible by the action of this very complicated enzyme. Here is a typical image:
The areas in dark grey and light grey are the areas on the enzyme where individual CO2 molecules are attracted and held. They are then transported to an area where the Carbon portion of CO2 is combined with other molecules to create a form of glucose.
Despite its huge importance in life, RuBisCO is, by enzyme standards, rather slow, with a turnover rate of between 3 and 10 CO2 molecules per second. Typical enzyme reaction rates are somewhere around 1/2 million molecules per second. So the first two severe limitations to this enzyme are:
1. Complicated and energy expensive molecule for a plant to manufacture, which may require a few weeks to adjust the levels.
2. Very slow reaction rate.
The central role of RuBisCO in the process of photosynthesis means that it must be tightly regulated, to ensure that it is active only where and when it should be. One important layer of this regulation is the activation of RuBisCO at the beginning of the day. During the night, the RuBisCO active sites are blocked by inhibitors. So, at the beginning of the photoperiod, the plant then has to re-activate the binding sites by removing the inhibitors from the CO2 binding sites. This means that there is another set of proteins which are sensitive to light. There are additional regulators to ensure that RuBisCO only operates at daytime. Magnesium typically increases as the chlorophyl molecules become active so the sensors monitor the movement of Mg++. So here is a third limitation:
3. Activation of the enzyme is very slow at the beginning of the photoperiod. The RuBisCO in algae is more efficient because algae have a mechanism which concentrates CO2 and holds it, so they respond very quickly to the daylight. This is why siestas are a very bad idea.