But, lots more variables to consider there.
Yes there are a ton of variables. In hope of turning this into a great thread I will add more info as I see it and encourage other to do so as well.
So far the info below is based on logic, not facts. These benefits are only valid if the split period if plants adapt to the split period and growth is the same as a straight period except for the things mentioned below.
Split photo period/"siesta" Pros/Cons
Pros:
-In a non CO2 injected tank(see note 1) or even DIY CO2 levels may bottom out during the mid day. Then the rest of the day they are struggling to get any/all CO2 that is available. Using the split period with 3-5hours of "siesta" will let the CO2 levels build back up since plants only uptake CO2 when the lights are on. So when the lights come back on they can continue growing at faster rates from the higher CO2 levels. Also the steadier CO2 levels when the lights are on should give plants the upper hand compared to algae that takes advantage of low or fluctuating levels.
-If you have low water movement in the tank, the siesta may also be a benefit as it will let plants and the water time to get more nutrients. If the water surrounding the plants is depleted. This could benefit any tanks that have low flow(even CO2 injected ones), but prob not as much difference in tanks that use substrate ferts.
Cons:
-Fluorescent bulbs wear out faster
Notes:
1:Not all non co2 tanks are the same. NPT(natural planted tanks) or similar that use some kind of "soil" for the substrate are very different from using a regular substrate. The main reason is that in a "soil" tank, the soil and decaying matter can be turned into CO2. So many feel that low water movement in a NPT is good as CO2 levels can get above ambient levels(has been tested in natural bodies of water). In a regular substrate tank, many feel that good surface agitation is good and will try to keep the co2 levels near ambient.
There may very well be a point which is too long for the "siesta", where plants will not adapt very well,so that will have to be considered.
So in a DIY or non CO2 injected tank, my opinion is that for the "siesta" to be used to the most benefit, the 1st lighting period should be even for a DIY CO2 tank, the period inbetween will be found by finding out how long it takes when the lights go out for the CO2 levels to build back up. If it takes to long to build back up, then the last lighting period should be shorter then the 1st.
If its non CO2 injected tank, then the CO2 will prob build up slower when the lights are off depending on the method. So you may need a longer "siesta" to give time for the CO2 levels to rise, it may also not have enough tie to rise to what it 1st was, In this casei ts prob best to have the last lighting period shorter than the 1st.
Please add your $.02