Answer:
David could be suffering from sleep deprivation, which may cause fatigue, tiredness, irritability, depression, or loss of focus. This could also be an affect of suprachiasmatic nucleus in which David would have a sleep schedule that's wrong or delayed. David may also be suffering from sleep apnea which can cause irritability or David could be skipping stage 3 sleep (in turn, making him almost just as tired as when he went to sleep) or skipping REM sleep which could cause memory issues.
And finally, David could also be experiencing an alcohol overdose, also known as a hangover. This would make David feel fatigued, irritable, and it could lead to depression. David has many symptoms that can link to many different things and while sleep deprivation, stage 3 sleep, REM sleep, sleep apnea, suprachiasmatic nucleus, and alcohol may be the cause, there's no way to know just from the information provided what could have caused David's symptoms.
Explanation:
