The website you quote makes it sound like the subconsciousness is a little man in your brain, with a book of knowledge, reviewing all the material during the night and "strengthening knowledge without your influence". Although the website doesn't properly support the idea about how post-training sleep might benefit memory consolidation, it does convey some "truth".
Gais et al. 1 studied the timing of learning, sleep and subsequent recall, and indeed found that when learning is immediately followed by sleep, this improves subsequent recall as compared to conditions in which learning was followed by wakefulness.
The task involved learning German words. In the picture below you can see the different conditions that were tested. In the first experiment (A) Gais tested whether sleep following learning or wakefulness following learning enhanced the consolidation of declarative memory (morning vs. evening). When comparing the different conditions, the evening-learning condition revealed a better performance than the morning condition.
To control for time-of-day effects a second experiment (B) was conducted, consisting of 2 evening conditions. One evening condition where learning was immediately followed by sleep and another evening condition where sleep was postponed until the morning hours. Again results favoured learning when it was immediately followed by sleep.
Some mechanisms by which the role of sleep following learning (and the role of sleep in learning in general) might be explained are "memory replay" during sleep (neuronal activity reflecting the learning of a certain task during the day, re-occuring during the night), and/or synaptic downscaling (the synaptic homeostasis hypothesis).
The synaptic homeostasis hypothesis... predicts that sleep may enhance
performance by global downscaling...
For more background information read the review of Tononi & Cirello 3, or a recent review by Ribeiro 4.
Now... what about these 15-20 minutes? Well I don't know where this advice comes from and don't know about scientific support for this claim. In the study of Gais the duration of learning (although not exactly mentioned) was "a few hours". I would say they are advantages and disadvantages to a learning period of 15-20 minutes as opposed to a longer learning period.
15-20 minutes rehearsal seems ideal in the sense that you'll be focusing on a limited amount of information. Strengthening this limited amount of information might increase their chances of being retained (I think this idea could be integrated with the synaptic downscaling hypothesis). When keeping study time or learning time shorter you might be able to increase focus. Keeping focus for a long time is more difficult and the efficiency by which one learns might decrease. However, you can also choose to learn for a longer time on a limited amount of information, thus increasing repetition and again increasing chances of the information being retained. It more or less depends on how you spend the time learning...
In addition, the above mentioned claims might depend on what type of information you want to retain. Studies might report contradicting results, depending on what type of material is to be learned. E.g. complex material requires more investment. Also learning requires time... one has to invest in understanding the material and there are different strategies by which one can try to learn material that aren't covered in the studies.
Personally I believe studies related to learning and sleep are really interesting to decipher what happens to the information we collect during the day. What happens during the part of the day where we are the least conscious of what's happening around us? When it comes to learning in itself however... I think it's more important to think about "how" you learn the material and how often you repeat learning the material, than the timing of learning in relation to sleep (e.g. paper on memory retrieval by Karpicke and Blunt 4).
- Gais, S. (2006). Sleep after learning aids memory recall. Learning & Memory, 13(3), 259–262. doi:10.1101/lm.132106
- Tononi, G., & Cirelli, C. (2006). Sleep function and synaptic homeostasis. Sleep Medicine Reviews, 10(1), 49–62. doi:10.1016/j.smrv.2005.05.002
- Ribeiro, S. (2012). Sleep and plasticity. Pflugers Archiv : European journal of physiology, 463(1), 111–120. doi:10.1007/s00424-011-1031-5
- Karpicke, J. D., & Blunt, J. R. (2011). Retrieval practice produces more learning than elaborative studying with concept mapping. Science (New York, NY), 331(6018), 772–775. doi:10.1126/science.1199327