Is there any research in this area, it seems "Sign in" is more common and hence more recommended.
In "Homepage Usability", Jakob Nielsen (together with Marie Tahir, 2002, p. 53) recommends the use of "sign in" / "sign out" over "log in" / "log out". This is empirically based on a survey of several large-scale websites and thus supports OP's "more common" argument.
Furthermore, I second @Dan Barak in that you should use "Register" or "Join [your-service-here]" as opposed to "sign up" in order to avoid unnecessary confusion. (I cannot recall whether Nielsen and Tahir had any recommendations regarding this issue, though.)
While I don't have a very strong opinion here, I would bear in mind:
- Sign In and Sign Up are quite close.
Users might click one instead of the other sometimes.
Either you make the difference more evident by location or graphics, or you could also use "Register or "Join" instead.
- Make sure you stay consistent with the log out vs. sign out.
I think that this article about the “Sign Up” button is interesting, the author changed the “Sign Up” button to “Try it Free” and clicks increased by 212%.
His thesis is that the standard “Sign Up” buttons don’t work because “they ask for blind commitment” and “do not offer any value”.
Visitors also “see common elements repeated on many sites” and “they begin unconsciously ignoring those elements (aka “habituation”)”.
- Tie it to your product. If you have a SaaS for trading bitcoins: “Start Trading Bitcoins.” If you have a marketplace for artists: “Start Selling Art.” This helps prevent the button from being overlooked.
- Give, don’t take. “Get Access” and “Sign Up” both lead to the same thing, but one makes the visitor feel they’re getting something, while the other doesn’t.
- Compel people to act. Use action verbs such as get, start, and try.
Of course there are many variables to consider (what kind of website is yours? Changing the label of the button increases clicks, but what about new subscriptions? etc.) but it may be worth having a look at it.
Here's a good overview on how a few popular sites are using Sign in, Log in etc. Login/Logout vs Sign In/Sign Out vs Log in/Sign out – A short roundup
Use Log in to avoid capture slips
I would be very careful with the "common usage" argument. For example: the use of sign up and sign in has a very pleasant symmetry which doubtless appeals to many people. Unfortunately, this symmetry reduces the difference by which the user recognizes the button she needs to just two letters. It's very easy to click sign up when you meant sign in.
Ultimately, the fact that everybody does it, doesn't mean that it's good UX. There's plenty of terrible UX patterns that are conventions. (Confirmation dialogs, anyone?)
For the reasons above, I would not use any option with sign in it: even if you go with "sign up/log in", the fact that it's so close to "sign in" means the user has to pay more attention, even if she makes the right choice in the end.
If you use "register/log in", there is no chance of confusion, and you lighten the cognitive load.
I agree consistency is key.
This means not just consistent within your own site but with the general web (if it's a web based app)
I believe you can't be too far from the 'norm' following the example of google, yahoo, etc.
Google: Sign Out, Sign In, "Create an account"
Yahoo: Sign Out, Sign In, "Sign up for a new account"
Just on a gut feeling - I would rather prefer using "Sign in", "Sign out" and "Register/Join/Create account" variation over "Log in".
Mostly because "Logging" something does not really convey the meaning of "entering" quite the same way as "sign in" does.
I can log any daily event, but that is just a mention of a fact while when I sign in at the door of an office building, I am giving my signature that I have entered the building and when I "sign out", I am also recording the fact that I am leaving the building...
I personally prefer the Log In / Sign Up combination. My justification for this is that the Sign In and Sign Up will confuse people, it becomes harder for them to find what each button means unless they reach the end of the word.
Log In is pretty standard and gets the job done and takea away the confusion as well.
As a fairly old Englishman I've grown up understanding that signing up meant you were entering some kind of contract requiring your signature. Often these contracts or agreements had some legal obligations attached so when ever I see "Sign up" it makes me nervous.
On the other hand ,"Sign in", to me means something completely different only used to indicate a visit. No legal or binding connections.
I think there is no definitive answer on this.
It really depends on your demographic.
If you demographic understands what Sign in is, its ok. However, there is a case in my home country where majority of people don't understand what "Sign in" means. The "Log in" is much better understanded. So when I changed the button to "Log in" and the users of that country specific website started to log in more often.
So the answer is use "Sign in" as a best guess used by Google/Yahoo, but check if you demographic does not confuse it and understands it well.
If the people in your country do not understand it well use "Log in"
Based on google trends, "login" is used 9 times more than "sign in".
Nevertheless, there are two countries that prefer "sign in" over "login": France and Turkey.
"sign in" is used 50% more than "log in".
"log on", "logon" and "sign on" are not widely used.
Although Jakob Nielsen recommended in 2002 the use of "sign in" / "sign out" over "log in" / "log out", the dialog principles of the ISO 9241-110 norm recommends that design should be in conformity with user expectations. In other words: if everyone is doing it, you should also do it.
Another important dialog principle is the error tolerance. Having "login" and "sign up" will avoid capture slips as recommended in Peter's answer
In summary: Use "Login" for the title of your pages and links, "Log in" in the buttons to log in and "Sign up" to create a new user account.
In light of the fact that there's so much subjective opinion on this topic, I'd like to call for standardization, perhaps by W3 as part of their WCAG accessibility guidelines. The lack of a standard makes this a real problem for blind and vision impaired users who typically use a search tool to find the login link (or button). Having to search in turn for "sign in", "log in", "signin", and "login" (all of which I've personally seen) is frustrating and time consuming.
I just had this same dilemma as well. In this instance, we're really isolated ourselves to English.
I ultimately took the shortest route by using Log In for http://simmerwp.com. "Log" is shorter than "sign" so that's what I let my decision ride on ultimately. As you point out, many social networks and other platforms use "Sign," and others use even more (longer) outlandish copy.
I think the placement, and icon selection are almost more important than the text, however. In the instance above you can see we went for a super minimalist design in the main navigation at full responsive width, and down around tablet and mobile widths, we have a single icon to combine these areas.
Cheers, and good luck!
Edit: To be honest, I don't know why we're not using just "Login" vs. "Log In," the former being shorter still. Welcome to any thoughts on that spelling as well.
The term 'log in' is, from what I see, considered somewhat 'antiquated' - When a few people I know hear the phrase log in, they claimed they first thought of the windows 95/98 log in screen.
I believe that it is just a stylistic change of no real importance.
Personally, I prefer 'log in', but then again I also use a beige telephone and an IBM keyboard that predates windows 95/98. So take that with a pinch of salt. I am of the impression that it is now seen as antiquated, in any case.
Problem with Log in is there is no Log up. You have to either use Create Account, Join or Sign up anyways. My thinking is that Login, Logout is mostly used in Desktop software and Sign up and Sign in look mostly website related.
However, I myself use Sign up and Sign in in my desktop software because it seems US population is used to mostly Sign up and Sign in.
Another confusion is between the use of 'Login' or 'Log in'. 'Sign up' and 'Login' makes sense too. However, in my view, Logout is better than Sign out for desktop software.
I would personally use "login", "sign out", and join. In my opinion, "join" is easier to distinguish than "sign up," and sounds better than "register". I would use "login" and "sign out" because I see them used more commonly than "sign in" or "log out". They are also easy to use and distinguish. This website even uses "Log In" and "Sign Up"!
As long as you use "sign in" or "log in" as opposed to "login", I don't think it really matters that much. Users understand both terms equally well from the results I've seen. I've never seen a person confused with where to log in when it says either term.
Definitely be consistent with log in/out or sign in/out either way. I'd also use register instead of sign up unless you have a good reason to use the latter; sign up and sign in are very similar at a glance.
Before anyone says anything about it in comments on this, login (and logon, logoff & logout) is bad because it is an adjective, not a verb. You don't login, you log in on the login page.
Sign up means "to register; to create an account". In computing, sign in and log in are synonyms. Both mean "to open a session with an account that is already created". There is one difference: the derived noun login "a username; a session under that username" exists, but there is no such noun as *sigin
In my opinion, We may use "Sign In" when a user "Signed" an agreement while registering to a website while on the "Sign up" Section. This means you hearby sign and enter the system.
Whereas "Login" says Ok create my Log and let me enter into the system.
I havent read any articles on this, this is just a wild guess as the names say it all.