Age to stay home alone overnight

Have you asked your teen how he/she feels? To me, gauging you teen's comfort level is key.  

The key word here is "responsible":  It sounds as though you trust your daughter, and I think 14 going on 15 is plenty old enough to stay by herself.  If you have an alarm system, even more so.  Ditto if there is a trusted neighbor or nearby adult she could call.  Basically, she needs to be look out for herself in an emergency until you could get home -- whether it's an earthquake or an exploding water heater or whatever else could go wrong.  Your coworker who said this is illegal is wrong. 

There's no California state law that sets a minimum age for this. The standards for child neglect or endangerment are, in general, a judgment call based on the overall circumstances (the child's age and maturity, how long and where and in what situation the child was left alone).  There are some states that have laws setting forth either a rule or a "guideline" for the minimum age to be left at home alone - it ranges from 6 to 14 - but I don't know of any that specifically address overnight vs during the day.  Handy reference here: http://www.freerangekids.com/laws/
Is a sitter unnecessary for your 14-15 year old while you work night shift?  It's really up to you to decide whether your kid is mature enough and how comfortable you are with the resources she would have available to her in the event of some problem or emergency.  For what it's worth, my son first stayed at home alone overnight at age 15, and it was fine, though it's not something we need to do often.

California, like almost every other state in the US, doesn't specify an age at which a child can be left at home alone. There is no bright line rule; that is, there isn't a law that says "child must be at least X years old to be left unattended." Theoretically, then, you could leave a child home alone at any age. However, if something happens - the kid has an accident, sets the drapes on fire, whatever - then you can be charged with neglect. 

For practical purposes, what this means is that you will have to make a determination about whether your child can be left home alone based on that child's level of maturity. There are certainly 12 year olds who can be left alone overnight (so long as they are comfortable - not if the child is afraid) because they are mature and responsible. On the other hand, there are 17 year olds who are not mature enough to be left alone overnight without risk. The American Academy of Pediatrics states that children of 11 or 12 are generally mature enough to be left at home alone for up to 3 hours during the day (not at night). The key is that the child knows what to do in case of an emergency: call a neighbor, call 911, whatever is appropriate.

When my daughter was growing up we started leaving her home alone for an hour or two when she was about 10. By 15 we felt comfortable leaving her for two days with one overnight, provided we had phone contact in case she needed us. Certainly by 15 she would have been embarrassed by the idea that she needed a babysitter herself, but I imagine some of her peers would have been afraid of being home alone.

One way to think about it is this: Is this child mature enough and responsible enough to babysit? It the answer is yes, then they should be able to take care of themselves.

 I'm sure you know your daughter well enough to make the right decision for her and yourself.

I am another single parent who is very interested in this question.  My mid-20s daughter began staying home alone while I worked night shifts at age 16.  She was a very responsible kid.  Currently my 14-year old son has a sitter from 6 - 9 pm, goes to bed, and older sister comes over later to stay overnight the nights I am at work.  My son gets himself up in the morning & is very responsible about morning routine and getting to school.  Occasionally the evening sitter cannot make it, and he manages just fine with a few phone calls from me.  My rules for the teen are:  no use of stove/oven (microwave is ok).  And no visitors.

It might be legal, but unless necessary probably not a good idea. Your friend apparently doesn't have a choice, so I don't think there is much to say about it.

On the other hand, if parents can avoid leaving teens alone at night, it would be wise to be there for them, or to hire a babysitter/companion. I teach high school and even "good" kids when left alone are unlikely to complete homework, or go to sleep at a reasonable hour. Other problems can develop when friends know they are alone. Also, teens may hide it, but they get lonely when there isn't someone around to be with them.