(Adds retailer comments, research on scale of problem, background)

April 13 (Reuters) - New York's attorney general has sent letters to 13 national retailers, including Gap Inc, Target Corp and JC Penney Co Inc, about "on-call shifts" in which workers are told whether to report to work a day or less before a scheduled shift.

Attorney General Eric Schneiderman's letters, sent on Friday, say on-call systems leave "too little time to make arrangements for family needs, let alone to find an alternative source of income to compensate for the lost pay" on days the employees are not called in to work.

A number of companies with stores in New York are requiring employees to check in by telephone, text message or email before a planned shift to see if their services are needed, Schneiderman wrote in the letters.

The system allows retailers to adjust staffing based on store traffic forecasts made by scheduling software. The companies can then reduce over-staffing and under-staffing.

His requests come as workers' advocates claim success in efforts to increase pay and benefits at fast food companies and national retailers, including recent raises of minimum wages by McDonald's Corp and Wal Mart Stores Inc.

Schneiderman said the "on-call" practice might violate the law in New York, where employers are subject to a rule that says employees who report for a scheduled shift on any day have to be paid for at least four hours at the basic minimum hourly wage.

Target said workers are informed of their schedules 10 days before the start of a work week and it does not employ "on-call" shifts. JC Penney said it has a policy against on-call scheduling. The Gap said it is committed to "sustainable scheduling practices" and is conducting research on the matter.

Worker advocates say unpredictable scheduling is one of the key challenges facing low-wage workers.

"One of reasons it is coming to light now is that people are organizing around it," said Tsedeye Gebreselassie, senior attorney at the National Employment Law Project.

He noted that a 2011 union-backed study of New York retail workers showed a fifth surveyed were required to always or frequently be available for on-call shifts.

Bills addressing on-call scheduling are currently being considered in the state legislatures of Massachusetts, Connecticut, Minnesota, Oregon and California, according to the Center for Popular Democracy, a worker advocacy group.

The U.S. Labor Department said it is aware of the on-call scheduling concerns and is looking into the matter.

"This is an important issue for workers struggling with work-life balance, especially for women," spokeswoman Tania Mejia said.

Schneiderman asked the retailers to provide details on the processes they follow to schedule on-call shifts, such as whether they use computerized systems and penalize employees who do not follow on-call procedures.

He also asked the companies for any analysis they might have conducted on cost savings associated with on-call shifts and the impact on workers' wellbeing. The companies have until May 4 to send in their responses.

The Gap said it was engaged in a research project with the UC Hastings College of Worklife Law to examine scheduling and productivity, and expects to receive some data in the fall of 2015.

"In the meantime, each of our brands also has been working to evaluate and refine their practices to make improvements," a spokeswoman for the retailer said.

Letters were also sent to Abercrombie & Fitch Co, J. Crew, L Brands Inc, Burlington Coat Factory, TJX Cos Inc , Urban Outfitters Inc, Crocs Inc, Ann Inc, Sears Holdings Corp and Williams-Sonoma Inc.

Sears and Ann Inc both said they do not use on-call scheduling. Representatives of the other retailers did not immediately respond to requests for comment.

(Reporting by Supriya Kurane and Siddharth Cavale in Bengaluru, Karen Freifeld and David Morgan in New York, Nathan Layne in Chicago, and Lisa Baertlein in Los Angeles. Editing by Anupama Dwivedi, Dan Grebler and Andre Grenon)