Sap studied economics at the University Tilburg between 1981 and 1989 where she specialized in political economy and philosophy. Between 1985 and 1988 she was a student assistant and coordinator at the department of development cooperation. She continued to work at the University of Amsterdam as research for three years. She studied the difference in pay between men and women. After that she became a research associate at the government Council for Emancipation. She initiatied "Out of the Margin": two international conferences on feminist economics in 1992 and 1998. This lead to a book "Out of the Margin. Feminist Perspectives on Economics", which she edited with Susan Feiner. Since 1993 she was active within the GreenLeft, as member of its economic committee.
In 1996 she started to work at the Ministry of Social Affairs and Employment, where she adviced top civil servants on emancipation and life planning policy (combining labour, care and education over one's life time), in addition to salary, income and pension policy.
In 2003 she left the ministry to become director of LEEFtijd (LIFEtime). She reoriented LEEFtijd on new issues such as the demographic developments in the Netherlands and life planning policy. In 2006 she co-authored the election program of the GreenLeft. She was put on the eight place of the list of the Tweede Kamer for the 2006 elections. The party only gained seven seats. In 2007 she became a member of the panel principles of the commitee Van Ojik, which would organize the discussion that will lead to a new manifesto of principles for the Greenleft. In 2008 she left LEEFtijd for a sabbatical. In August 2008 it was announced that she would replace Wijnand Duyvendak, who left the Tweede Kamer.
Sap is a specialist on social-economic issues. She values economic independence as an important condition for freedom. She seeks to stimulate emancipation and fight social exclusion. She wants to work on practical policies which would implement GreenLeft ideals such as self-realization, fairness and durability.