The terms drowsiness, sleepiness and [hypnagogic period][1] are often used to describe the same thing in the general domain, but but in the scientific domain there are important neurophysiological and psychophysiological differences.

<h2>Hypnagogic period</h2>
This is the neurophysiological and psychophysiological transition period from the wake to sleep state.

<h2>Drowsiness</h2>
This is the state of the transition period from the wake to sleep state **that can be objectively measured**, for example, by EEG (please see [Hori et al., 1994][2])

Typical tests that describe the quality and quantity of the drowsyness are [Multiple Sleep Latency Test (MSLT)][3] and Maintenance of Wakefulness Test (MWT) ([Doghramji et al., 1997][4]).

<h2>Sleepiness</h2>
This is the **subjective state of the subject/patient**. It can also be qualified and quantified, but only by rating scales such as the [Stanford Sleepiness Scale (SSS)][5], the [Epworth Sleepiness Scale (ESS)][6] (See also [Johns, (1991)][7]), the Karolinska Sleepiness Scale ([Åkerstedt & Gillberg, 1990][8]), etc.

<h2>References</h2>
Åkerstedt, T., & Gillberg, M. (1990).  Subjective and objective sleepiness in the active individual, *The International Journal of Neuroscience*, 52(1-2): pp. 29–37)<br>
DOI: [10.3109/0020745900899424][9] PMID: [2265922][8]

Doghramji, K., Mitler, M. M., Sangal, R. B., Shapiro, C., Taylor, S., Walsleben, J., Belisle, C. Erman, M. K., Hayduk, R., Hosn, R., O’Malley, E. B., Sangal, J. M., Schutte, S. L., & Youakima, J. M. (1997). A normative study of the maintenance of wakefulness test (MWT), *Electroencephalography and clinical neurophysiology* 103(5): pp. 554–562.<br>
DOI: [10.1016/S0013-4694(97)00010-2][10] PMCID: [PMC2424234][4]

Hori, T., Hayashi, M., & Morikawa, T. (1994). Topographical EEG changes and the hypnagogic experience. In R. D. Ogilvie & J. R. Harsh (Eds.), *Sleep onset: Normal and abnormal processes* (pp. 237-253).<br>
DOI: [10.1037/10166-014][2]

Johns, M. W. (1991). A new method for measuring daytime sleepiness: the Epworth sleepiness scale, *Sleep* 14(6): pp. 540-545<br>
DOI: [10.1093/sleep/14.6.540][11] PMID: [1798888][7]


  [1]: https://en.wikipedia.org/wiki/Hypnagogia
  [2]: https://doi.org/10.1037/10166-014
  [3]: https://en.wikipedia.org/wiki/Multiple_Sleep_Latency_Test
  [4]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2424234
  [5]: https://web.stanford.edu/~dement/sss.html
  [6]: http://www.britishsnoring.co.uk/sleep_apnoea/epworth_sleepiness_scale.php
  [7]: https://www.ncbi.nlm.nih.gov/m/pubmed/1798888
  [8]: https://www.ncbi.nlm.nih.gov/m/pubmed/2265922/
  [9]: https://doi.org/10.3109/0020745900899424
  [10]: https://doi.org/10.1016/S0013-4694(97)00010-2
  [11]: https://doi.org/10.1093/sleep/14.6.540