Do we have technology to ethically measure the changes of interstitial space in sleeping humans? Lulu Xie et al write in their paper Sleep Drives Metabolite Clearance from the Adult Brain:


  Using real-time assessments of tetramethylammonium diffusion and two-photon imaging in live mice, we show that natural sleep or anesthesia are associated with a 60% increase in the interstitial space


Do we have technology to do the similar measurements in humans and not only in mice? Have those measurements been done?
It is possible to extract samples of interstitial fluid in humans.
The method has been described in detail.
Is it ethically permissible to extract the entire contents of the interstitial space? No. It is vital for survival.


Technically, it seems possible to perform the procedure before and after sleep. I am not sure if this has been done though. To what extent this is an invasive procedure I do not know, but it appears to involve surgery.


In addition, local interstitual fluid pressure (IFP) can be
measured in humans under anesthesia.


Both methods pose ethical challenges and it appears that these methods are relevant in clinical settings, where the cost-to-benefit analysis would suggest a benefit to the patient.