Difference between drowsiness/sedation and sleepiness I have been trying to understand the difference between drowsiness/sedation and sleepiness. The only article I was able to find that partly answers my question is "The difference between sedation and sleep" on ipnos.com.

I have experienced both, and they feel distinctly different. Sleepiness feels like a drive, but drowsiness/sedation feels like a state. My personal experience has been that sleeping when drowsy/sedated does not relieve the drowsiness/sedation, but sleeping when sleepy resolves the sleepiness.

Back to the question: what is the neurological/neurobiological/psychopharmacological difference between sleepiness and drowsiness/sedation, and why does sleep resolve the one but not the other?
Sedation

This is a forced state of drowsiness or sleep and as indicated in your Ipnos link, it will not necessarily help with drowsiness or sleepiness brought about due to problems with your circadian rhythm

Sedation is


  the reduction of irritability or agitation by administration of sedative drugs, generally to facilitate a medical procedure or diagnostic procedure.


Drowsiness and Sleepiness

The terms drowsiness, sleepiness and hypnagogic period 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.

Hypnagogic period

This is the neurophysiological and psychophysiological transition period from the wake to sleep state.

Drowsiness

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)

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

Sleepiness

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), the Epworth Sleepiness Scale (ESS) (See also Johns, (1991)), the Karolinska Sleepiness Scale (Åkerstedt &amp; Gillberg, 1990), etc.

References

Åkerstedt, T., &amp; Gillberg, M. (1990).  Subjective and objective sleepiness in the active individual, The International Journal of Neuroscience, 52(1-2): pp. 29–37)
DOI: 10.3109/0020745900899424 PMID: 2265922

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., &amp; Youakima, J. M. (1997). A normative study of the maintenance of wakefulness test (MWT), Electroencephalography and clinical neurophysiology 103(5): pp. 554–562.
DOI: 10.1016/S0013-4694(97)00010-2 PMCID: PMC2424234

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

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