What can be done always depends on what caused it in the first place. Once you have a cause, you have a target for what to do about it. Lack of sleep can cause this. Eye inflammation from dry eyes, allergies and conditions like conjunctivitis as well as eye strain (from phones, reading, computer screens/games are common causes. Acute illness and fever like with the flu and medications, alcohol and drug use/abuse are potential causes. Systemic illnesses like autoimmune disease and diabetes can be associated with glassy eyes. 

Always address with lifestyle modifications first – sleep, inflammation, eye strain, certain drugs/alcohol and allergic exposure are all modifiable through daily changes. Systemic diseases are impacted by lifestyle changes as well and can be managed or resolved completely.