Colonists have several status indicators that reflect how they are doing. You should probably monitor colonist status and make sure it is above acceptable levels.
When a status indicator is low, a warning sign will appear on top of that colonist indicating what's wrong.
This is a measure of the physical health of a colonist, if it goes too low, the colonist will die.
Colonist can be affected by a variety of conditions. Some of them are deadly, some are not, some require medical treatment, some are just minor nuisances.
Condition that can occur due to mining or other kinds of accidents.
Severe condition usually as a result of exposure to solar flares or industrial accidents.
Condition that sometimes can be brought into the base by visitors or new colonists.
Not usually serious, but it is contagious, and prevents colonists from working.
Severe condition that occurs as a result of being hit by firearms.
Condition that occurs as a result of not eating enough variety of food.
This is a measure of how well fed a colonist is, if it goes too low, Health will start to drop, and the colonist will eventually die.
This is a measure of how thirsty a colonist is, if it goes too low, Health will start to drop, and colonist will eventually die.
Colonists can drink water at a Drinking Fountain, that can be placed in the Canteen or in the Multi-Dome. Make sure that the number of Fountains is proportionate to the number of Colonists. If you see too many thirst icons, you need to place more Fountains.
This measures the Oxygen level of a colonist, if it goes too low, Health will start to drop, and colonist will eventually die.
When indoors, colonists need to be in a structure with enough oxygen in it.
This is a measure of how sleepy a colonist is. If it goes too low, colonists will be less effective at work, and they will stop working altogether if it drops below a certain level.
Colonists can sleep in a Bunk in the Dorm . They will try do this at night and keep a routine, but if they can't either because they need to work or there is no space, they will do it at any time.
Another way of recovering sleep is in a proper Bed in a Cabin. This recovers sleep faster and also recovers morale.
Be aware that sleepiness affects your Colony's welfare status. 
This is a measure of the psychological health of a colonist. If it goes low, colonists will be less effective at work, and they will stop working altogether if it drops below a certain level.
Working and carrying stuff around decreases morale.
Other events like disasters, or people dying will also impact morale.
Larger populations have an adverse impact on morale.
Various objects in the base, like plants, video screens, exercise facilities, drinks or individual beds allow colonists to recover morale. The most effective is the bar (if you can afford to spend the food resources), since all other morale boosting devices have a cooldown.
In late game it is suggested that you use cabins because of the advantages they have over dorms despite being more expensive.