It's obvious that people will move more when awake or doing exercise compared to being asleep or resting, however actigraphy provides a quantitative way to measure that. Therefore actigraphy is useful for studying sleep-wake cycles, activity-rest cycles and circadian rhythms. They have been shown to be reliable in determining when a subject is awake or asleep, so that's why they are generally used for that. Although there still needs to be more standardization and consistency of analysis and reporting in scientific literature to make comparison between studies more useful ([Berger et al., 2008][1]).

In order for actigraphy to be useful for understanding brain activity while awake, there needs to be a way to monitor brain activity. When exercise is involved methods such as fMRI, PET, TMS are not suitable since subjects need to stay still. EEG and EMG are really the only options. [Winkler et al., 2003][2] has correlated short-term actigraphy with awake EEG patterns, and results suggest that they can be used to indicate CNS arousal. It probably doesn't affect the results, but you should note that the study was with psychiatric patients who were taking medication. It would have been more thorough to do this with normal population as a control to ensure generalizability. The software/algorithm they used was "Actiwatch Sleep Analysis 2001" (Cambridge Neurotechnology). Maybe you can look that up to find out more, although I'm not sure the algorithm would be published. 


  [1]: http://www.sciencedirect.com/science/article/pii/S0885392408001127
  [2]: http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowAbstract&ProduktNr=224082&ArtikelNr=74634